Fig. 231.
Fig.231.—Side view of skull of Cape Jumping Hare (Pedetes caffer). ×3⁄5.AS, Alisphenoid;Ex.O, exoccipital;Fr, frontal;L, lachrymal;Ma, malar;Mx, maxilla;Na, nasal;OS, orbito-sphenoid;Pa, parietal;Perpoints to the large supratympanic or mastoid bulla;PMx, premaxilla;Sq, squamosal;Ty, tympanic. (From Flower'sOsteology.)
The most distinct anatomical characteristic of the Rodents concerns the teeth. They are without exception entirely deprived of canines. Thus there is a long diastema between the incisors and the molars. Another peculiarity is, that in many cases the dentition is absolutely monophyodont. In such forms as the Muridae there seems to be no milk dentition at all. In that family there are only three molars; but in other types where there are four, five, or six molars, the first one, two, or three, as the case may be, have milk predecessors, and may thus be termed premolars. This has been definitely proved to be the case in the common Rabbit, which has the unusually large number of six grinding teeth in each half of the upper jaw when adult. The first three of these have milk forerunners. On the other hand the existence of four molars does not apparently always argue that the first is a premolar; for Sir W. Flower found that inHydrochoerus,[316]none of the teeth had any forerunners, at any rate so far as could be detected from the examination of a very young animal. The Rabbit appears to be also exceptional, in that the second incisor of the upper jaw and the incisor of the lower jaw have milk forerunners. In any case the tendency towards monophyodontism is peculiarly well-marked in this group of mammals. The incisors of Rodents are as a rule in each jaw a single pair of long and strong teeth, which grow from persistent pulps, andgrow to a very great length, extending back within the jaw to near the hinder part of the skull. These teeth are reinforced in the upper jaw by a small second pair in the Lagomorpha only. The incisors are chisel-shaped, and often brown or yellow upon the outer face, as is the case also with some Insectivores. This peculiar shape, and their strength, renders them especially capable of the gnawing action which characterises the Rodents. It has been pointed out that where the incisors are wider than thick, the gnawing powers are feebly developed; and that on the contrary, where these teeth are thicker than wide, the animals are good gnawers. The incisors have often an anterior groove, or it may be grooves.
Fig. 232.
Fig.232.—Molar teeth of Rodents.A, of Capybara (Hydrochoerus);B, of Squirrel (Sciurus);C, ofCtenodactylus. (After Tullberg.)
The cheek teeth vary in number from two (Hydromys) to six (Rabbit) on each side of the two jaws. Four is the prevailing number outside the large division of the Rat-like Rodents. They are often set at an angle to the horizontal plane of the jaw, looking outwards and obliquely to its longitudinal axis; the individual teeth too are not unfrequently bowed in form, reminding us of those ofToxodon. This of course only occurs in those genera which have hypselodont teeth. The pattern of the teeth varies much, and the different forms recall the teeth of more than one other group of mammals. They are either bunodont or lophodont. In many cases the tooth is encircled with a ridge of enamel, which is either almost simple or has a more complicated contour; such teeth are by no means unsuggestive of the Toxodonts. Some of the lophodont molars are by no means unlike those of the Proboscidea. InSciurus vulgaristheencircling ridge is broken up into tubercles, which gives to the tooth a striking likeness to those ofOrnithorhynchus. Other genera have teeth like those of many Ungulates. It has been shown by Sir J. Tomes[317]that the minute structure of the enamel differs in different groups of Rodents.
The skull shows certain primitive characters. In the first place there is no distinction between the orbital and the temporal fossa.[318]The sutures between the bones retain their distinctness for very long. Other characteristic features are the following:—The nasals are large, and so are the paroccipital processes. The palate in front of the molars is not distinct from the sides of the skull, its edge gradually becoming rounded off above. It is also very narrow. The premaxillae are large in relation to the great incisors. There is often a very much enlarged infra-orbital foramen through which passes a part of the masseter muscle. The jugal bone lies in the middle of the zygomatic arch, which is complete and enormously enlarged in the Spotted Cavy (Coelogenys paca). As in many Marsupials, the jugal bone sometimes extends backwards to the glenoid cavity, where the lower jaw articulates. It is usually said with an absolute want of accuracy that the cerebral hemispheres of the Rodents are smooth and without convolutions. This error has been repeated again and again in text-books. As a matter of fact the cerebral hemispheres of many forms are quite well convoluted,[319]the degree of furrowing corresponding, as in so many groups of mammals, with the size of the animal. This at any rate is generally true, though the large Beaver with its scant convolutions is an exception. The smaller forms, such asMus,Sciurus,Dipus, andCricetusare quite smooth-brained. The best furrowed brain of any Rodent which has been examined is that of the hugeHydrochoerus. The Sylvian fissure is very generally not pronounced; but is particularly well-marked inLagostomus. In all, or in most, Rodents the hemispheres are separated by an interval from the cerebellum, the optic lobes being visible between the two.
The mouth cavity of this group of mammals is divided into two chambers by a hairy ingrowth behind the incisors; this arrangement is useful for animals which use their strong incisorsas gnawing and excavating tools as well as for the purposes of alimentation; for it allows of substances being gnawed away without the products of the chisel-like action being taken into the hinder cavity of the mouth. The Rodents have for the most part a simple stomach of normal form; but in a few this is complicated by a marked constriction, which divides the cardiac from the pyloric portions. The Hamster, for example, is thus characterised. In all the members of the order, with the exception of the Dormice and some allied forms, the caecum is large and often sacculated. In some forms (e.g.Arvicola,Myodes,Cuniculus) the large intestine is coiled upon itself in a spiral way—a state of affairs strongly suggestive of Ruminants.
The Rodents are primarily divisible into two great groups, the Simplicidentata and the Duplicidentata, characterised mainly by the upper incisor teeth. In the former there is but one pair of these teeth; in the latter a second smaller pair lie behind the former.
The Anomaluri are separated by Thomas and others from this section as an equal section, while by Tullberg they are grouped withPedetes.
Fam. 1. Anomaluridae.—The genusAnomalurussuggests at first sight the Flying Squirrels of Asia,Pteromys. It is, however, an entirely African genus, and is to be distinguished from the Asiatic Rodents by a series of scales at the root of the tail, imbricated, keeled, and forming possibly a "climbing organ." This character serves also to distinguish the present genus fromSciuropterus. The cartilage, moreover, which supports the patagium springs from the elbow. There are four molars in each half of each jaw. The eyes and ears are large. There are five fingers and toes, but the thumb is small, though provided with a nail. The sternum has seven joints, and nine ribs reach it. The clavicle is strong. Huet, who has recently monographed the genus,[320]allows six species. The species vary in size.
Anomalurus peliappears, according to Mr. W. H. Adams,[321]to be a common species on the Gold Coast; it is coloured black and white, but in spite of the warning which this colour should convey, is considered by the perhaps rather omnivorous native as "the greatest delicacy." The animal is nocturnal, but affects only bright moonlight nights. Their "flying" consists of a jump from a high branch to a lower one, after which they reascend the tree to a point of vantage for another jump. They are said to feed upon nuts; but Tullberg only found the remains of leaves in the stomach.
Idiurusis a lately-described genus allied toAnomalurus. There are at any rate two species,I. zenkeriandI. macrotis. The thumb is more reduced than inAnomalurus, and the fibula, contrary to what is found in that genus and in most Sciuromorphs, is fused with the tibia below.
A third genus, very recently described and allied to both the foregoing, isAëthurus. It is a native of the French Congo,[322]and differs by the absence of flying membranes. It has, however, the pad of large scales. There is but one species,A. glirinus. It has a black bushy tail. The postorbital processes of the frontals are totally wanting—there are not even the traces to be seen inAnomalurus. The thumb has vanished. If we are to compareAnomaluruswith the Squirrels then, thinks Mr. de Winton, the present genus is probably diurnal by reason of the want of flying membranes.
Fam. 2. Sciuridae.—The Squirrels, genusSciurus, are world-wide in range, the Australian region and the island of Madagascar being alone excepted.
The eyes and the ears are large; the tail is of course long and bushy. The fore-feet have an inconspicuous thumb; the hind-feet have four toes. The soles of the fore-feet are naked or furry, those of the hind-feet are hairy. There are twelve or thirteen dorsal vertebrae, and in correspondence seven or six lumbars. The caudal vertebrae may be as many as twenty-five. In the skull the frontals are broad, and there are longpostorbital processes. The infra-orbital foramen is, as a rule, not large, but is increased in size in a few forms. The number of separate pieces of bone in the sternum is five. The molars of the upper jaw are five, but the first is very small and soon drops out.
The Squirrels are often rather brilliantly coloured. The ChineseSc. castaneiventrishas grey fur with a rich chestnut-coloured under surface. The Malabar Squirrel,Sc. maximus, as its name implies, a large animal, has a deep reddish or chestnut-coloured fur above, which becomes yellow below. The "Common Squirrel," "the lytill squerell full of besynesse," which is the Squirrel of this country, is brownish red on the upper parts and white below. It ranges from this country as far east as Japan. Like many other Rodents the Squirrel likes animal food and will eat both eggs and young birds. "Camel's hair" brushes are made from this animal. The genusTamias, almost exclusively North American in range, is included by Dr. Forsyth Major[323]in this genus, which then consists of considerably over one hundred species.
The Ethiopian Ground Squirrels, genusXerus, have a more elongated skull thanSciurus, and the postorbital processes are shorter. The feet are not hairy.
Nannosciurusforms a perfectly distinct genus of Squirrels. These "Pygmy Squirrels" differ in possessing a very elongated "face" and in the very broad frontal region. The teeth are unlike those ofSciurusin certain features, and have been especially compared by Forsyth Major to those of the Dormice. Four species of this genus are Malayan; one is West African.
The BorneanRheithrosciurus macrotisis the only species of its genus. The genus may be distinguished by the exceedingly brachyodont molars, this feature being more marked in this genus than in all other Squirrels. It is called the "Groove-toothed Squirrel," from the "seven to ten minute parallel vertical grooves running down the front face of its incisors."[324]
The genusSpermophilusincludes a large number (forty or so) of Palaearctic and Nearctic animals known as Sousliks. The ears are small; there are cheek pouches as inTamias. The general aspect of the animal is like that of a Marmot, and they bridge over the exceedingly narrow gap which separates the Marmots from the true Squirrels. Anatomically the skull is like that ofArctomys; the molars are five in the upper and four in the lower jaw. The caecum is relatively speaking very small; the measurements in a specimen ofS. tredecimlineatus, dissected by Dr. Tullberg, were: small intestine, 580 mm.; large intestine, 170 mm.; and caecum, 27 mm. InTamiasalso the caecum is not greatly developed. These animals are burrowing in habit.
The Prairie-dogs, genusCynomys, of which the best-known species is perhapsC. ludovicianus, are very like the Squirrels, but they are not arboreal creatures; they live in burrows on the ground, as their vernacular name denotes. The genus is entirely North American, and four species have been differentiated.
The Prairie-dog or Prairie-marmot is some 10 inches to one foot in length. The tail is no more than 2 inches. The ears are very small; the thumb is fully developed and bears a claw. The measurements of the various sections of the intestine are the following:—Small intestine, 860 mm.; large intestine, 690 mm.; caecum, 75 mm. Thus the caecum is not large comparatively speaking. These animals dig burrows on grassy plains which they share with the Ground Owl (Speotyto cunicularis) and with Rattlesnakes, all three species appearing to live in perfect amity. Probably the Owls use the conveniently-constructed burrows, and the Rattlesnakes come there to look after the young of both.
Fig. 233.
Fig.233.—Long-tailed Marmot.Arctomys caudatus.×1⁄7.
Closely allied to the last are the Marmots, genusArctomys. They differ in the rudimentary character of the thumb and in the longer tail. The eyes and ears are small. The distribution of the genus is Nearctic and Palaearctic. There are ten species ofthe genus. The Alpine Marmot,A. marmotta, is familiar to most persons. The animal lives high up in the Alps, and when danger threatens it gives vent to a shrill whistle. It hibernates in the winter, and as many as ten to fifteen animals may be found closely packed together in a single, carefully-lined burrow.
The only other European species isA. bobac, the Siberian Marmot, which occurs in the extreme east of Europe, and is also Asiatic. There are four North American species, including the Quebec Marmot,A. monax.
Fig. 234.
Fig.234.—Flying Squirrel.Pteromys alborufus.×1⁄5.
The genusPteromys(of which the proper name, antedatingPteromysby five years, appears to bePetaurista) is confined to the Oriental region, where there are a dozen species or so. The limbs are united by a parachute extending to the toes, and supported anteriorly by a cartilage attached to the wrists. There are also membranes anteriorly uniting the fore-limbs to the neck, and posteriorly uniting the hind-limbs to the root of the tail and a trifle beyond. The skull and the dental formula are as in Sciurus, but the pattern of the molars, which is much complicated, seems to argue a different mode of nutrition. There are twelve pairs of ribs. The large intestine (inP. petaurista) is very nearly as long as the small, and the caecum is also "colossal"; the measurements in an individual of the species named were: small intestine, 670 mm.; caecum, 320 mm.; large intestine, 650 mm.The caecum is disposed in a spiral. The teats are three pairs, non-inguinal in position.
The size of these squirrels is 16 to 18 inches exclusive of the tail, which is longer. These animals can make an exceedingly long jump with the help of their flying membrane. Nearly eighty yards is the longest distance given for these aerial excursions. It is stated that they are able to steer themselves to some extent while in the air. As appears to be the case with so many Rodents, these animals feed largely upon beetles and other insects, besides bark, nuts, etc.
The allied genusSciuropterushas a much wider range. It extends into the Palaearctic region and into North America, besides being found in India. There is here no membrane reaching to the tail. The palms and soles are furry. The caecum is very much shorter, and so is the large intestine. The latter, inS. volucella, is not more than one-third of the length of the small intestine. In other features there are no remarkable differences in structure, save that the mammae, always three pairs, may be inguinal.
Of the genusEupetaurus[325]but a single species is known, which is limited to high altitudes at Gilgit and perhaps in Thibet. Its principal difference from the other genera of Flying Squirrels is that the molars are hypselodont instead of brachyodont. The interfemoral membrane is rudimentary or wanting. The one species isE. cinereus. It is thought to live "on rocks, perhaps among precipices." Dr. Tullberg attributes the hypselodont teeth to the fact that the mosses upon which it is believed to feed may have much sand and earth intermingled, which would naturally lead to a more rapid wearing away of the teeth, and hence a need for a good supply of dental tissue to meet this destruction.
Fam. 3. Castoridae.—This, the third family of the Sciuromorpha, contains but one genus,Castor, the Beaver, with at most two species, one North American, the other European. This large Rodent has small eyes and ears, as befits an aquatic animal, and the tail is exceedingly broad and covered with scales; the transverse processes of the caudal vertebrae, in order better to support the thick tissues lying outside them, are divided in the middle of the series into two. The hind-feet are much larger than the fore-feet, and are more webbed than in any other aquatic Rodent.
In the skull the infraorbital foramen is small as in Squirrels. The postorbital process has practically vanished. The four molars stand out laterally from the jaws. The incisors, as might be surmised from the habits, are particularly strong. The stomach has near the entrance of the oesophagus a glandular patch, which seems to be like that of the Wombat (see p.144). In both sexes the cloaca is very distinct and comparatively deep.
The two species of the genus areC. canadensisandC. fiber. The latter is of course the European species, which is now found in several of the large rivers of Europe, such as the Danube and the Rhone. But it is everywhere getting scarce, and limited to quite small and isolated colonies.
In this country it is absolutely extinct and has been since before the historic period. There is apparently no documentary evidence of its survival down to this period. But the numerous names of places which are called from this animal illustrate its former prevalence. Examples of such names are Beverley in Yorkshire, and Barbourne or Beaverbourne in Worcestershire. In Wales, however, Beavers seem to have persisted longer. But they were rare in the Principality for a hundred years or so before the Norman Conquest. The king Howel Dda, who died in 948A.D., fixed the price of a Beaver skin at 120 pence, the skins of Stag, Wolf, and Fox being worth only 8 pence apiece. The Beaver was called by the Welsh "Llost-llyddan," which means "broad-tail." Its existence in the country is handed down in the name of Llyn-ar-afange, which means Beaver lake. The last positive record of the Beaver in Wales seems to be the statement of Giraldus Cambrensis that in 1188 the animal was still to be found in the river Teivy in Cardiganshire. In Scotland the Beaver is said to have continued down to a later date. Ireland it never reached. The remains of this animal by their abundance show the former prevalence ofC. fiberin this country. It is known from the fens of Cambridgeshire, and from superficial deposits elsewhere. The Thames formerly had its Beavers, and apparently it was widely spread through the country generally.
The Beaver not only furnishes collars and cuffs for coats; it was used, as every one knows, to provide hats. But the usefulness of the animal by no means ended here in the eyes of ourforefathers. The Rev. Edward Topsell observed that "for giving great ease unto the gowt the skinnes of beavers burned with drie oynions" are excellent. Castorein as a drug, if not in actual use, has quite recently been a part of the pharmacopoeia. It is derived from the anal glands common to this and other Rodents, and indeed many other mammals.
A large extinct form of Beaver isTrogontherium,[326]found in the "Forest-bed" of Cromer. The skull is about one-fourth longer than that ofCastor. It has a less inflated bulla, and slightly more pronounced postorbital processes thanCastor. The third molar (fourth grinding tooth) is relatively larger than inCastor, and has a rather more folded crown. The foramen magnum is more triangular.
Fam. 4. Haplodontidae.—A separate family seems to be required for the genusHaplodon, whose characters will therefore be merged with those of its family. It is to be distinguished from most other Squirrel-like creatures by the fact that there is no postorbital process to the frontal. The molar teeth are five in the upper and four in the lower jaw. The Sewellel,H. rufus, like the other species of the genus (H. major), is found in North America west of the Rocky Mountains. It has the habit of the Prairie-marmot, and has a short tail, only moderately long ears, and five-toed feet. Tullberg is of opinion that this animal nearly represents the ancestral form of the Squirrel tribe.
This subdivision of the Rodents contains, according to Mr. Thomas's recent estimate,[327]no less than 102 genera. It is therefore obviously impossible to do more than refer to some of the more interesting of these. This group is again divided into the following families:—
(1) Gliridae, including the Dormice.
(2) Muridae, the Rats, Mice, Gerbilles, Australian Water-rats, Hamster.
(3) Bathyergidae, Cape Mole, etc.
(4) Spalacidae, Bamboo Rats.
(5) Geomyidae, Pouched Rats.
(6) Heteromyidae, Kangaroo Rats.
(7) Dipodidae, Jerboas.
(8) Pedetidae.
The Gliridae have no caecum, so usual in the Rodentia. It is true that all the genera have not been dissected, but it is known that in the true Dormice, as well as in the genusPlatacanthomys, a caecum is absent.
Apart from these few exceptions the Mouse-like Rodents all possess a caecum, though it is often not very large. They are all smallish animals, and are modified to a great variety of habit and habitat. There are burrowing, swimming, and climbing forms. The group is universal in range, even including the Australian region, in which they are the only Rodents.
Fam. 1. Gliridae.—This family, also called Myoxidae,[328]includes the Dormice, and is entirely an Old-World family, absent only from the Malagasy region. Its most important differential character is the total absence of the caecum and of any sharp boundary between the small and large intestine. The molars are usually four. The eyes and ears are well developed.
The genusMuscardinusincludes only the Common Dormouse,M. arellanarius. This small creature, 3 inches long with a tail of 2½ inches, is, of course, a well-known inhabitant of this country. It is also found all over Europe. It is not particularly abundant in this country, and a good specimen is said to be worth half a guinea. As the specific name denotes, it lives largely on hazel nuts; but it will also suck eggs and devour insects. The animal makes a "nest" in the form of a hollow ball. Its hibernation is well known, and has also given rise to the German name ("Schläfer") of the group. It was well known to Aristotle, who gave or adopted the nameἘλειὸςfor the animal. Its winter sleep—suggestive of death—and its revivification in the spring gave the Bishop of Salamis, Epiphanius, an argument for the resurrection of man. The fur was reckoned in Pliny's time a remedy for paralysis and also for disease of the ears.
The genusMyoxusincludes also but a single species,M. glis, the so-called "Fat Dormouse" of the Continent. It has noglandular swelling at the base of the oesophagus, such as occurs in the last genus and inGraphiurus. OfGraphiurusthere are thirteen species, all African in range. The genus does not differ widely from the last. There is, however, a glandular region of the oesophagus.Eliomysis the last genus of typical Dormice. It is Palaearctic in range.
Platacanthomys, of a Dormouse-like form, has like other Dormice a long tail, on which the long coarse hairs are arranged in two rows on opposite sides towards the tip; it is represented by a single species,P. lasiurus, from the Malabar coast. It is arboreal in habit. The fur is mingled with flattened spines. The molars are reduced to three on each side of each jaw. This form has been bandied about between the "Mice" and the "Dormice"; but Mr. Thomas's discovery of the absence of the caecum argues strongly in favour of its correct location among the Gliridae.Typhlomysis an allied genus, also from the Oriental region. This and the last are placed in a special sub-family of the Gliridae, Platacanthomyinae, by Mr. Thomas.
Fam. 2. Muridae.—This family, that of the Rats and Mice in a wide sense, is the most extensive family of Rodents. In it Mr. Thomas includes no less than seventy-six genera. The molars are generally three. The tail is fairly long, or very long, and the soles of the feet are naked.
Sub-Fam. 1. Murinae.—The true Rats and Mice may be considered to form a sub-family, Murinae. The genusMus, including the Rats and Mice in the limited sense of the word, contains about 130 species. They are exclusively Old World in range, being only absent from the Island of Madagascar. In the New World there are no species of the restricted genusMus. The eyes and the ears are large; the pollex is rudimentary, and bears a nail instead of a claw. The tail is largely scaly. All the members of the genus are small animals, some quite minute. In this country there are five species[329]of the genus, viz. the Harvest Mouse,M. minutus, which has a body only 2½ inches long with an equally long tail. It is the smallest of British quadrupeds with the exception of the Lesser Shrew. The Wood Mouse,M. sylvaticus, is about twice the size; it differs also from the last species in that itfrequents barns, and is thus sometimes mistaken for the Common Mouse, from which, however, it is to be distinguished by its coloration and longer ears. The latter,M. musculus, is too familiar to need much description. A curious variety of it has occurred. This has a thickened and a folded skin like that of a Rhinoceros, and the hair has disappeared. The Black Rat,M. rattus, is like a large Mouse, and is smaller and blacker in colour than the "Hanoverian Rat." It is sometimes called the "Old English Rat," but seems nevertheless to be not a truly indigenous Rodent. It has been so defeated by competition with the Hanoverian Rat that it is now not a common species in this country.
The Hanoverian or Brown Rat,M. decumanus, is a larger and a browner animal than the last. It is very widely distributed through the globe, no doubt largely on account of the fact that it is readily transported by man. The same is the case with the Common Mouse, whose real origin must be a matter of doubt. The original home of the Brown Eat is thought by Dr. Blanford to be Mongolia. There is so far a justification for the name "Hanoverian Rat" that the animal seems to have reached this country about the year 1728. But there is no reason for calling it, as is sometimes done, the Norway Rat.
Some members of this genus, whose fur is interspersed with spines, or which are quite spiny, have been separated as a genus,Leggada, which, however, is not generally allowed.
Closely allied again isChiruromys, which has a strongly prehensile tail, a feature which is not common among the Myomorpha, thoughDendromys, a tree-frequenting form, andMus minutus, already spoken of, show the same character. Many Mice seem to have prehensile tails, which they can curl round branches; but it is not so fully developed as in the species just named.
A number of other genera are referable to the true Mice, the sub-family Murinae of Thomas's classification. The Syrian and AfricanAcomyshas very spiny fur, so much so that "when it has its spines erected it is almost indistinguishable at the first glance from a diminutive hedgehog." The generaCricetomys,Malacomys,Lophuromys,Saccostomus,Dasymysare restricted to the Ethiopian region.Nesokiais Oriental, reachingalso the Palaearctic region.Vandeleuria,Chiropodomys,Batomys,Carpomysare Oriental, the last two being confined to the Philippines.
Another peculiar Philippine genus isPhlaeomys, of large size, and allied to it isCrateromys, originally confounded with it.Batomys grantiis also confined to Luzon. Its molars are three, like those of the also restricted and PhilippineCarpomys melanurus, which is an arboreal form. There is a second species,C. phaeurus.
Phlaeomysis placed, however, by Mr. Thomas in a distinct sub-family of its own,Phlaeomyinae, and is removed from the Murinae.
Hapalomys, with but one species, is Burmese.Pithecochirusis Javanese and Sumatran.Conilurus(also known asHapalotis) is a genus containing species which are termed Jerboa Rats, on account of their mode of progression. They are desert and Australian forms. There are sixteen species.
Fig. 235.
Fig.235.—Spiny Mouse.Acomys cahirinus.× ½.
Mastacomys, with one species, is limited to Tasmania.Uromys, with some eight species, is from Queensland, and inhabits also the Aru Islands and the Solomon Islands. The CelebesianEchiothrix, orCraurothrixas it should apparently properly be called, is another genus containing but a single species.Golundais both Oriental and Ethiopian, one species occurring in each region. The beautiful little striped Barbary Mice,Arviacanthis(orIsomys), are African, north as well as tropical.
The genusSaccostomusresembles the Hamsters in the presence of cheek pouches. Its teeth, however, are Murine. It agrees withSteatomysin the comparatively short tail. The caecum is rather long.
Sub-Fam. 2. Hydromyinae.—The genusHydromys,[330]of which there are several species, the best known beingH. chrysogaster, is an exclusively Australian form, and is aquatic in habit. It is a foot or so in length, and has a fairly long tail. The fore- and hind-limbs are webbed, in correspondence with its habits. The Australian Water-Rat is black, with an admixture of golden-coloured hairs dorsally and golden colour below, with a lighter median stripe. The thumb is small, and the webbing of the hands is not so marked as is that of the feet. The molars are only two in each half of each jaw. The caecum is rather small, the measurements of the alimentary canal being: small intestine, 895 mm.; large intestine, 278 mm.; caecum, 70 mm. Allied to the last isXeromys, a genus which is also Australian, but limited to Queensland. It has been established by Mr. Thomas,[331]who discovered that it has the same reduced formula asHydromys.Xeromys, however, is not an aquatic animal, and has unwebbed feet.
In the Luzon highlands Mr. Whitehead has discovered, and Mr. Thomas quite recently described,[332]a number of peculiar Rodents. Of these the generaChrotomys,Celaenomys, andCrunomysare allied to the Australian and New GuineaHydromys.
Chrotomys whiteheadiis unusual among Muridae, in its coloration being marked by a pale stripe down the back. The creature is the size of the Black Rat (Mus rattus). It is terrestrial not aquatic in habit, in spite of its likeness toHydromys. The molars, however, are 3/3.
Crunomys fallaxis more likeHydromys. It has, however, three molars, as in the last genus. But the skull has the flattened form characteristic ofHydromysas opposed toMus.
LikeBatomys,Celaenomys silaceusis also somewhat intermediate betweenHydromysandMus. It is described as very Shrew-like in appearance, and has a very pointed muzzle. Its habits Mr. Whitehead is "quite unable even to guess at." LikeHydromysandXeromysthis Rodent has but two molars.
Sub-Fam. 3. Rhynchomyinae.—The genusRhynchomys, containing but one species,Rh. soricioides(of Thomas), is also, as both its generic and specific names imply, a somewhat Shrew-like form in external aspect. The skull, too, is Insectivore-like in itselongation, and the lower incisors are worn to needle-like points. The two molars are excessively minute, and thus the always large gap in the jaws is greatly exaggerated. It is suggested that this Rat is an insect-eater, but nothing positive is known.
Sub-Fam. 4. Gerbillinae.—The Gerbilles form another sub-family, Gerbillinae, of the Muridae, or a family, according to some. The best-known genus isGerbillus, including the Gerbilles proper. These animals are Old World in range, belonging to the three regions of that part of the world. There are a large number of species in the genus, over thirty. They have a Jerboa-like form, with rather long hind-limbs and a long and hairy tail. But the hind- as well as the fore-feet are five-toed. The molar teeth have no trace of tubercles, but only transverse lamellae of enamel. The incisors are orange; they are white inDipus.Gerbillus pyramidumis 90 mm. long, with a tail of 125 mm. The ears are long, 13 mm. The tail has longer hairs at the tip.
Fig. 236.
Fig.236.—Gerbille.Gerbillus aegyptius.× ½.
Psammomysis in some respects different. The tail is shorter than inGerbillus; its length in an individual of 165 mm. was 130 mm. As inGerbillusthere are four pairs of teats, two pectoral and two inguinal. This genus is exclusively Palaearctic in range.Merioneshas a range co-extensive with that ofGerbillus.
Pachyuromysis an Ethiopian genus with a short tail. As the generic name denotes, the tail is not only short but thick and fleshy.
Sub-Fam. 5. Otomyinae.—The allied genera,OtomysandOreinomys, are Ethiopian.Otomys unisulcatushas a tail shorter than the body, the measurements of a female of this species being 137 mm. with a tail of 87 mm. The ear is long, whence the name; it measured in this specimen 20 mm.
Sub-Fam. 6. Dendromyinae.—The genusDeomysis an African form, consisting of only one species from the Congo region.D. ferrugineushas a reddish colour as its name implies; the soles are quite naked and the tail is long and slender. It is considerably longer than the body, measuring (minus a fragment of the tip) 172 mm., while the body is 125 mm. long. The characters of the molar teeth, which are three, are intermediate in their form between those of the true Rats and those of the Hamsters.
Dendromysis also Ethiopian in range. There are several species.D. mesomelasis a smallish creature, 60 mm. long, with a tail of 90 mm.
Steatomysis another African genus, allied to the last. Its tail, however, is only half the length of the body. The two remaining genera areMalacothrixandLimacomys. Their range is African.
Sub-Fam. 7. Lophiomyinae.—Allied to the Hamsters is the singular East African genusLophiomys, with only one species,L. imhausi, of Milne-Edwards.[333]The size is between that of a Rabbit and of a Guinea-pig. The stomach is curved and somewhat intestiniform. It has been termed the Crested Rat on account of the "prominent crest of stiff hair running down the back." The fingers and toes are five, and the very long tail is clad with hair longer than that upon the body generally. The pollex is rudimentary, and the hallux is opposable.
The most remarkable structural feature in this genus concerns the skull, and on account of this it has been regarded as the type of a separate family. The temporal fossa behind the eye is covered over by a complete bony plate, formed by a downgrowth of the parietal, meeting an upgrowth from the malar; this singular arrangement of the bones recalls the conditions which obtain in turtles. The whole skull, moreover, is covered with symmetrically disposed granulations, such as are found in no other mammal; it suggests rather the skull of certain fish. It is believed that the bony plate already referred to is not really a portion of the bones of which it appears to be a prolongation, but merely an ossification of fasciae in this region. The atlas is granulated like the skull; there are sixteen pairs of ribs and a feeble clavicle. The molars are three, and of a peculiar form.They have, in the case of the first three, transverse ridges, from which stand up two sharp and long tubercles. The other teeth have two ridges. The incisors are pale yellow. The shape of the teeth and the smallness of the caecum suggest that this Rodent is not so purely a vegetarian as others, and that it nourishes itself largely upon insects.
Sub-Fam. 8. Microtinae.—The Voles or Water-Rats form a distinct group of Murine animals, to which the sub-family name of Microtinae has been applied from the genusMicrotus(more generally known asArvicola), a genus which includes the Water-Rat and Field-Voles of this country. This genus has short ears, and a short and hairy tail. Its build is stouter and clumsier than that of the Rats. The genus is confined to the Palaearctic and the Nearctic regions. In this country there are three species. The best known is the Water-Vole or Water-Rat,M. amphibius, which has been seen by most people, and which frequents streams, ponds, and canals. The feet, curiously enough, are not webbed, which seems to argue the recent adoption of an aquatic life. Mr. Trevor-Battye has remarked that this animal, when swimming at leisure, uses its hind-limbs only, carrying the fore pair at the sides like a Seal. The Bank-Vole,M. glareolus, is rather a local species in this country. It is a terrestrial Vole, and burrows. The Field-Vole,M. agrestis, has become notorious on account of the "plagues," to which its immense numbers have on occasions given rise. It is the smallest species, and has a greyish-brown fur like the Water-Vole, the Bank-Vole being redder. To give an idea of the cost of the depredations of this animal, Mr. Scherren quotes[334]a farmer who gave evidence before the Agricultural Commission to the effect that, putting the damage of one Vole at two pence, the amount of loss suffered on a farm of 6500 acres in two years would be £50,000!
The genusFibercomes very near the last. It is a North American genus. The hind-feet are slightly webbed; the tail is a trifle shorter than the body, and is compressed and scaly, with scattered hairs. The thumb is short, but with a fully-developed claw. As in the last genus, the small and large intestines are roughly of the same length, and the caecum is about one-fourth of the length of either. It is known as the "Musquash."
OfFiber zibethicus, or rather a closely-allied form,F. osoyoosensis, from Lake Osoyoos near the Rockies, Mr. Lord writes[335]that it constructs for itself a house of bulrushes built up from the bottom in 3 or 4 feet of water. It is dome-shaped, and rises about a foot out of the water. "If a dead or badly-wounded duck be left on the pool, it is at once seized on, towed into the house, and doomed." Thus it appears that this Rodent, like so many others, is largely carnivorous. It has also been asserted that it eats fish.
Neofiberis an allied genus, North American in range. The species,N. alleni, is compared, as regards outward form, with the Water-Vole,M. amphibius. It has, however, a shorter tail.
Another very well-known member of this sub-family is the Lemming. The name, however, applies to two quite distinct genera. The genusCuniculus, including the Banded Lemming,C. torquatus, is an inhabitant of North America, Siberia, and Greenland. The tail is short, its length being 12 mm. as against a body length of 101 mm. The feet are furred beneath, a not unusual state of affairs in Arctic mammals. The ears are very slight. The thumb is well developed, and bears a claw.
InMyodes, on the other hand, which is not so markedly an Arctic animal, though occurring in both Palaearctic and Nearctic regions, the ears are rather bigger, though still smaller than those ofMicrotus. The under surfaces of the feet are similarly furred. The tail is also short. It is commonly said that the two genera are to be distinguished by the furred feet ofCuniculus, and by the absence of fur in the present genus. That, according to Tullberg, does not appear to be the case. The differences are thus so much reduced that it seems almost unnecessary to retain the two genera. The best known species ofMyodesis of course the Scandinavian Lemming,M. lemmus. This animal used to occur in this country in Pleistocene times (as did alsoC. torquatus), and recently Dr. Gadow has found remains with skins attached in caves in Portugal. It may still survive in the mountains of the Peninsula.
The actual habitat of the Lemming in Scandinavia is the great tablelands, 3000 feet high in the centre. The migrations do not take place with regularity; even twenty years may elapse before the appearance in cultivated lands of those countlesshordes so familiar (as far as their description is concerned) to everybody. The Lemmings do not return from their exodus. They die from various causes, including combats with one another. Their chief foes, however, are Wolves and Gluttons, Buzzards and Ravens, Owls and Skuas, which batten on the migrant hordes. Their sudden increase in numbers recalls the similar increase at times of the Field-Vole, to which reference has already been made.
Ellobiusis an Old-World genus, which leads a "Talpine" life, and has in consequence rudimentary external ears and very small eyes. The tail is short. Contrary to what might be expected from its mode of life, the claws upon the digits are not strong.
The remaining genera of Vole-like Murines arePhenacomysandSynaptomysfrom North America, and Siphneus from Palaearctic Asia.Evotomysis one of those genera which are common to both the Palaearctic and the Nearctic regions, but the bulk of the species are North American.
Sub-Fam. 9. Sigmodontinae.—This is the name given to another sub-family of Murine Rodents, a group which includes the Hamsters in the Old World as well as a large number of South American genera of Rat-like animals. Of these latter there are a very large number, the bulk of the group being American.
The Hamsters, genusCricetus, as it is usually called, although apparently the correct name is Hamster, are Old-World forms of Pouched Rats. The Common Hamster,C. frumentarius, is about 210 mm. long, with a tail of 58 mm. It has cheek pouches. The small and the large intestines are not very unequal in length, and the caecum is fairly large, being about one-sixth to one-seventh of the length of either. It is a purely vegetable-feeding creature, and in Germany where it occurs (and from which language its vernacular name is derived), hibernates during the winter in its burrow, having previously surrounded itself with a great accumulation of food carried thither.
To North America are peculiar the generaOnychomys,Sigmodon, andPeromyscus. The genusSigmodon, the Cotton Rats, reaches Central America, and even gets a little farther south. The other two genera, though mainly North American, also extend their range to the south.Onychomyshas hairyfoot-pads, a state of affairs which characterises a number of these Rodents.
The generaMegalomys,Chilomys,Reithrodontomys,Eligmodontia,Nectomys,Rhipidomys,Tylomys,Holochilus,Reithrodon,Phyllotis,Scapteromys,Acodon,Oxymycterus,Ichthyomys,Blarinomys,Notiomysare South American forms.OryzomysandRheithrodontomysare common to both parts of the New World.
The genusIchthyomysis remarkable on account of its un-Rodent-like habits and of certain associated structural changes.I. stolzmanniwas obtained from Mount Chanchamays in Peru at an altitude of 3000 feet; it is a habitual fish-eater, and lives in streams. Another species,I. hydrobates, was formerly referred toHabrothrix. The skull shows likenesses to that of the AustralianHydromys; but the most marked characters of adaptation are those of the teeth and caecum. The cutting edges of the upper incisors form a reversedVof obvious use in holding a slippery fish. The caecum is much reduced, short, and narrow. The general Otter-like shape of the creature is largely due to its flattened head, though its "size and general proportions are much as in the common Black Rat."[336]
This sub-family contains a number of genera from Madagascar, viz.Brachytarsomys,Nesomys,Hallomys,Brachyuromys,Hypogeomys,Gymnuromys, andEliurus.
Sub-Fam. 10. Neotominae.—The last sub-family of the Muridae is that of the Neotominae, containing the North American generaNeotoma,Xenomys,Hodomys, andNelsonia.
Fam. 3. Bathyergidae.—This family contains several genera which consist of subterranean forms. All these Rodents agree in a number of characters, of which the principal are as follows:—
The eyes are very small, and the external ears are reduced to the merest fringe of skin round the aural aperture. The legs are short, as is the tail; the hair-covering is reduced—a reduction which finds its culmination in the nearly nudeHeterocephalus. Being burrowing creatures, a number of other modifications in accordance with this mode of life are to be seen in their structure. The upper incisors stand out in front of the closed lips, and prevent the entrance of earth. For the same reasonthe nostrils are small, and the forehead but little expanded between them.
The genusBathyerguscontains but a single species, the Cape Mole-Rat, which is found in Southern Africa; it is of moderate size, not exceeding a small Rabbit in dimensions. On the fore-limbs are exceedingly long claws, of which that borne by the second finger is the longest, and the claw of the thumb the shortest. The hind-feet have by no means such long claws. The scratching and burrowing is naturally chiefly effected by the fore-limbs. The small and large intestines are of equal length, and each is rather more than six times the length of the caecum; in these measurements the present genus differs from the next.
Georhychus.—Of this African genus there are about ten species. The claws are not so long as in the last genus, but there are, as inBathyergus, four molar teeth on each half of each jaw. The intestinal measurements in an example ofG. capensiswere: small intestine, 25 inches; caecum, 4 inches; large intestine, 15 inches.
The genusMyoscalopsorHeliphohius(also with an African range) has six back teeth on each side. A number of species sometimes referred to the last genus are placed here by Mr. Thomas. The claws are small.
One of the most remarkable genera of this family is the littleHeterocephalusfrom Abyssinia and Somaliland. As Mr. Thomas justly remarks,[337]it "is a peculiar-looking little creature, about the size of the Common Mouse, but looking almost more like a tiny hairless puppy on account of its nearly naked skin, small eyes, and peculiar physiognomy." Though apparently naked, there are numerous scattered hairs over the entire body, and the toes are fringed with stiffish hairs, which must be advantageous to a burrowing animal. There are two species,H. glaber(originally described by Rüppell), andH. phillipsii, of which our knowledge is due to Mr. Thomas. The length of the entire creature including the tail is not more than 134 mm., both species being approximately of the same dimensions. Mr. Lort Phillips, the discoverer of the species which bears his name, writes "that this little creature, called 'Farumfer' by the Somali, throws up in places groups of miniature craters, which exactly resemble volcanoes in active eruption. When the little beasts were atwork, I used frequently to watch them, and found that the loose earth from their excavations was brought to the bottom of the crater, and sent with great force into the air in a succession of rapid jerks, and that they themselves never ventured forth from the shelter of the burrows."[338]
Fam. 4. Spalacidae.—"The Spalacidae," observes Dr. Blanford, "are sometimes called rodent moles, and resemble a mole in general aspect, having cylindrical bodies, short limbs, small eyes and ears, large claws, and a short or rudimentary tail." The existence of a spiral valve in the caecum may perhaps characterise this family; but it has at present only been found in the two generaSpalaxandRhizomys.
Spalaxhas inconspicuous eyes and external ears. The tail is totally absent. The lower incisors are more developed than in other Rodents; they project in a bony sheath beyond the posterior end of the ramus of the lower jaw. The scapula is long and narrow. The large intestine is half the length of the small intestine. The animal seems to have only two pairs of teats, one pectoral the other inguinal.
Spalax typhlusof Egypt, which is probably not different from the European form, makes extensive burrows, some of the branches being even 30 to 40 yards in length. In a "domical chamber," situated along the course of one of these burrows, Dr. Anderson found no less than 68 bulbs stored up. Its eyes are mere black specks among the muscles, but they appear, however, to have a proper organisation. There are altogether eight species of the genus, which is entirely Palaearctic in its range.
The genusRhizomys, including a number of species known as Bamboo Rats, is purely Oriental in range.Rh. sumatrensisreaches a length of 19 inches; the better-known species,Rh. badius, is at most only 9 inches in length—in both cases the measurements are exclusive of the tail, which is a quarter to one-third of the length of the body, and is not scaly but nearly naked, with a few scattered hairs. The molars are three, and the incisors usually orange in colour; but sometimes the upper incisors are white as inRh. badius. There are thirteen dorsal vertebrae. InRh. pruinosusthe large intestine is considerably longer than the small intestine; the lengths of the two sections of the gut are 42 and 30 inches respectively. In anotherspecies the large intestine is slightly shorter than the small intestine. InRh. badiusthe two parts of the gut are almost exactly equal in length. There are three pairs of inguinal and two pairs of pectoral teats. The nameRhizomysappears to have been given to the animals of this genus for the reason that they feed largely on roots. They burrow, and, like many other burrowing animals, feed in the evening. As is the case with other forms,Rhizomysis said to burrow with the assistance of its teeth as well as of its claws.