Stenopora exilis
Fig. 41.—a,Stenopora exilis(Dawson).b,Chaetetes tumidus(Edwards and Haine). Carboniferous.
The true Stony Corals (Anthozoa) are as yet unknown in the Cambrian. They entered on the stage in immense abundance in the Siluro-Cambrian, where considerable limestones are largely composed of their remains, mixed, however, and sometimes overpowered with those of Bryozoa and Hydroids. An ordinary coral, such as those of which coral reefs are built—the red coral, used for ornament is not quite similar—is the skeleton of an animal constructed on the plan of a sea anemone; with a central stomach surrounded by radiating chambers, and having above a crown of tentacles. The stony coral surrounds and protects the soft body of the animal, and may either be a single cell, for one animal, or an aggregation ofsuch cells, constituting a rounded or branching mass. The modern star coral, represented inFig. 42, is an instance of the latter condition. It shows nineteen or twenty animals, each with a central mouth and fringe of short tentacles, aggregated together, and two of them showing the spontaneous division by which the number of animals in the mass is progressively increased. The living coral shows only the soft animals and the animal matter connecting them; but if dead there would be a white stony mass with a star-like cell or depression corresponding to each animal.
Living Anthozoan Coral.
Fig. 42.—Living Anthozoan Coral (Astræa).
In their general plan, the oldest Corals were precisely of this character, but they presented some differences in detail, which have caused them to be divided into two groups, which are eminently characteristic of the Palæozoic age—the tabulate or floored corals, and the rugose or wrinkled corals. In the former (Fig. 43) the cells are usually small and thin-walled, often hexagonal, like a honeycomb, and are floored across at intervals with tabulæ or horizontal plates. A few modern corals present a similar arrangement,14but this kind of structure was far more prevalent in the Palæozoic. In the second type the animals are usually larger and often solitary, the cell has strongly marked radiating plates, while the horizontal floors are absent or subordinate, and there is usually a thick external rind or outer coat (Figs. 44, 45). In general plan, theserugose corals closely resemble those of our modern reefs; but they differ in their details of structure, and only a very few modern forms from the deep sea are regarded as actual modern representatives.15One curious point of difference is that their radiating laminæ begin with four, and increase by multiples of that number, while in modern corals the numbers are six and multiples of six; a change of mathematical relation not easily accounted for, and which assimilates them to Hydroids on theone hand, and to a higher group, the Alcyonids, on the other, both of which prefer four and eight to six, or have had these numbers chosen for them. In the Mesozoic period the tabulate and rugose corals were replaced by others, the porous and solid corals of the modern seas; but, in so far as we know, the animals producing these, though differing in some details, were neither more nor less elevated than their predecessors, and they took up precisely the same rôle as reef-builders in the sea, though with probably more tendency to the accumulation of great masses of coral limestone in particular spots.
Tabulate Corals.
Fig. 43.—Tabulate Corals.
a,Halisites, andb,Favosites. Upper Silurian.
Rugose Coral.
Fig. 44.—Rugose Coral (Heliophyllum Halli). Devonian.
Zaphrentis prolifica.
Fig. 44a.—Zaphrentis prolifica(Billings). Devonian.
Rugose Corals.
Fig. 45.—Rugose Corals.
a,Zaphrentis Minas(Dn.), andb,Cyathophyllum Billingsi(Dn.). Carboniferous.
Leaving the corals, we may turn to the sea-stars and seaurchins. These merely put in an appearance in the Early Cambrian, but become vastly multiplied in the Silurian, where the stalked feather stars (Crinoids) (Fig. 46) seem to have covered great areas of sea-bottom, and multiplied so rapidly that thick sheets of limestone are largely made up of the fragments of their skeletons. The ordinary star-fishes appear first in the Silurian (Fig. 47). The sea-urchins begin in the Upper Silurian, the early species having numerous and loosely attached plates, like some of those now found in the deep sea16(Fig. 48).
Modern Crinoid.
Fig. 46.—Modern Crinoid (Rhisocrinus Lofotensis).—After Sars.
Palaeaster Niagarensis.
Palaechinus ellipticus
Fig. 47.—Palæaster Niagarensis(Hall). One of the oldest star fishes.
Fig. 48.—Palæchinus ellipticus(McCoy). One of the oldest types of sea-urchins.
The most curious history in this group is that of the feather-stars. In the Early Cambrian they are represented by a few species known to us only in fragments, and these belong to a humble group (Cystideans) resembling the larval or immature condition of the higher Crinoids.Fig. 49shows one of these animals of somewhat later age. They have few or rudimentary arms and short stalks, and want the beautiful radial symmetry of the typical star-fishes. In the Silurian these creatures are reinforced by a vast number of beautiful and perfect feather-stars (Figs. 50, 51). These continue to increase in number and beauty, and apparently culminate in the Mesozoic, where gigantic forms exist, some of them probably having more complicated skeletons, in so far as number of distinct parts is concerned, than any other animals. Buckland has calculated that in a crinoid similar to that inFig. 52there are no less than 150,000 little bones, and 300,000 contractile bundles of fibres to move them. In the modern seas the feather-stars have somewhat dwindled both in numbers and complexity, and are mostly confined to the depths of the ocean. On the other hand, the various types of ordinary star-fishes and sea-urchins have increased in number and importance. We thus find in this group a certain advance and improvement from the Cystideans of the Early Palæozoic to the sea-urchins and their allies. This advance is not, however, along one line for theCystideans continue unimproved to the end. The Crinoids culminate in the Mesozoic, and are not known to give origin to anything higher. The star-fishes and sea-urchins commence independently, before the culmination of the Crinoids, and, though greatly increased in number and variety, still adhere very closely to their original types.
Pleurocystites squamosus.
Heterocrinus simplex.
Body of Glyptocrinus.
Fig. 49.—Pleurocystites squamosus. Siluro-Cambrian. After Billings.
Fig. 50.—Heterocrinus simplex(Meek). One of the least complex crinoids of that period. Siluro-Cambrian.
Fig. 51.—Body ofGlyptocrinus. Siluro-Cambrian.
The great sub-kingdom of the Mollusca, including the bivalve and univalve shell-fishes, makes its first appearance in the Cambrian, where its earliest representatives belong to a group, the Arm-bearers or Lamp shells (Brachiopods), held by some to be allied to worms as much as to mollusks. The oldest of all these shells are allies of the modernLingulæ(Fig. 54), some ofthe earliest of which are shown inFig. 55. The modernLingulais protected by a delicate two-valved shell, composed, unlike that of most other mollusks, of phosphate of lime or bone earth. It lives on sand-banks, attached by its long flexible stalk, which it buries like a root in the bottom. Its food consists of microscopic organisms, drifted to its mouth by cilia placed on two arm-like processes, from which the group derives its name. In the modern world about one hundred species of Brachiopods are known, belonging to about twenty genera, some of which differ considerably from the Lingulæ. The genusTerebratula, represented atFig. 56, is one of the most common modern as well as fossil forms, and has the valves unequal, with a round opening in one of them for the stalk, which is attached to some hard object, and there is an internal shelly loop for supporting the arms.
Extracrinus Briareus.
Pentacrinus caput-medusae.
Fig. 52.—Extracrinus Briareus. Reduced. Jurassic.
Fig. 53.—Pentacrinus caput-medusæ. Reduced. Modern.
Lingula anatina.
Cambrian and Silurian Lingulae.
Fig. 54.—Lingula anatina. With flexible muscular stalk. Modern.
Fig. 55.—Cambrian and Silurian Lingulæ.
a,Lingulella Matthewi(Hartt). Acadian group.b,Lingula quadrata(Hall). Siluro-Cambrian.c,Lingulella prima(Hall). Potsdam.d,Lingulella antiqua(Hall). Potsdam.
These curious, and in the modern seas, exceptional shells, were dominant in the Palæozoic period. Upwards of three thousand fossil species are known, of which a large proportion belong to the Cambrian and Silurian, nine genera appearing in the Cambrian, and no less than fifty-two in the Silurian. The history of these creatures is very remarkable. The Lingulæ, which are the first to appear, continue unchanged and with the same phosphatic shells to the present day. Morse, who has carefully studied an American species, remarks in illustration of this, that it is exceedingly tenacious of life, bearing much change of depth, temperature, etc., without being destroyed. The genusDiscina, which is nearly as old, also continues throughout geological time. The genusOrthis(Fig. 57), which appears at the same time with the last, becomes vastly abundant in Silurian times, but dies out altogether before the end of the Palæozoic.Rhynchonella(Fig. 58), which comes in a little later, near thebeginning of the Siluro-Cambrian, continues to this day.SpiriferandProductus(Figs. 59 and 60) appear later, and die out at the close of the Palæozoic. So strange and inscrutable are the fortunes of these animals, which on the whole have lost in the battle of life, that their place in nature is vastly less important than it was. It has been suggested that if any group of creatures could throw light upon the theory of descent with modification, it would be these; but Davidson, who has perhaps studied them more thoroughly than any other naturalist,found them as silent on the subject as the sponges or the corals. In a series of papers published in theGeological Magazine, a short time before his death, he remarked as follows:
Terebratula sacculus.
Fig. 56.—Terebratula sacculus(Martin). Carboniferous.
Brachiopods; genus Orthis.
Fig. 57.—Brachiopods; genusOrthis.
a,O. Billingsi(Hartt). Lower Cambrian.b,O. pectinella(Hall). Siluro-Cambrian.c,O. lynx(Eichwald). Siluro-Cambrian.
Rhynchonella increbrescens
Fig. 58.—Rhynchonella increbrescens(Hall). Siluro-Cambrian.
Spirifer mucronatus
Fig. 59.—Spirifer mucronatus(Conrad). Devonian.
Athyris subtilita.
Fig.59a.—Athyris subtilita(Hall). Carboniferous.
a,b, Exteriors.c, Interior, showing spirals.
“We find that the large number of genera made their first appearance during the Palæozoic periods, and since they have been decreasing in number to the present period. We will leave out of question the species, for they vary so little that it is often very difficult to trace really good distinctive characters between them; it is different with the genera, as they are, or should be, founded on much greater and more permanent distinctions. Thus, for example, the familySpiriferidæincludes genera which are all characterised by a calcified spiral lamina for the support of the brachial appendages; and, however varied these may be, they always retain the distinctive characters of the group from their first appearance to their extinction. TheBrachiopodist labours under the difficulties of not being able to determine what are the simplest, or which are the highest families into which either of the two great groups of his favourite class is divided; so far, then, he is unable to point out any evidence favouring progressive development in it. But, confining himself to species, he sees often before him great varietal changes, so much so as to make it difficult for him to define the species; and it leads him to the belief that such groups were not of independent origin, as was universally thought before Darwin published his great work on theOrigin of Species. But in this respect the Brachiopoda reveal nothing more than other groups of the organic kingdoms.
Productus cora.
Fig. 60.—Productus cora(D’Orbigny). Carboniferous.
“Now, although certain genera, such asTerebratula,Rhynchonella,Crania, andDiscina, have enjoyed a very considerable geological existence, there are genera, such asStringocephalus,Uncites,Porambonites,Koninckina, and several others, which made their appearance very suddenly and without any warning; after a while they disappeared in a similar abrupt manner, having enjoyed a comparatively short existence. They are all possessed of such marked and distinctive internal characters that we cannot trace between them and associated or synchronous genera any evidence of their being either modifications of one or the other, or of being the result of descent with modification. Therefore, although far from denying the possibility or probability of the correctness of the Darwinian theory, I could not conscientiously affirm that the Brachiopoda, as far asI am at present acquainted with them, would be of much service in proving it. The subject is worthy of the continued and serious attention of every well-informed man of science. The sublime Creator of the universe has bestowed on him a thinking mind; therefore all that can be discovered is legitimate. Science has this advantage, that it is continually on the advance, and is ever ready to correct its errors when fresh light or new discoveries make such necessary.” The late Joachim Barrande, the great palæontologist of Bohemia, bears similar testimony.
Group of Older Palaeozoic Lamellibranchs.
Fig. 61.—Group of Older Palæozoic Lamellibranchs.—After Billings.
1,Cucullea opima. 2,Nucula oblonga. 3,Nucula lineata. 4,Cypricardia truncata. 5,Tellina ovata. 6,Nucula bellatula. 7,Modiola concentrica.
The ordinary bivalves, like the mussels and cockles, now so very plentiful on our coasts, are rare in the Cambrian and Silurian, and for the first time make a somewhat conspicuous appearance in the Upper Silurian and Devonian. But from the first they resemble very closely their modern successors, though on the whole neither so large nor so ornate (Fig. 61). Their fortunes have thus been precisely the opposite of those ofthe Brachiopods, though in neither case is there very marked elevation or deterioration in the individual animals. A very similar statement may be made as to the sea-snails, whether the curious winged snails (Pteropods) or the ordinary crawlers (Gastropods). The former come in early, and are represented by Palæozoic forms finer than any now extant. The genusConularia(Fig. 62) presents some Silurian species six inches or more in length, which are giants in comparison with any now living. The forms of more ordinary Gastropods from the Silurian represented inFig. 63will suffice to show that their styles are not very dissimilar from those still extant.17As inthe case of the ordinary bivalves, however, the modern Gastropods much exceed in numbers and magnitude those of the Palæozoic.
Conularia planicostata.
Fig. 62.—Conularia planicostata(Dn.). A Carboniferous Pteropod.
Silurian Sea-snails. Canada.
Fig. 63.—Silurian Sea-snails. Canada.
a,Murchisonia bicincta(Hall).b,Pleurotomaria umbilicatula(Hall).c,Murchisonia gracilis(Hall).d,Bellerophon sulcatinus(Billings).
The highest group of Mollusks, represented in the modern ocean by the Nautili and Cuttle-fishes, has a history so strange and eventful, and so different from what might have been anticipated, that it perhaps deserves a more detailed notice, more especially as Barrande has recently directed marked attention to it in his magnificent work on the Palæontology of Bohemia.
The Cuttle-fishes and Squids and their allies are, in the modern seas, a most important group (Fig. 64). The great numbers in which the smaller species appear on many coasts, and the immense size and formidable character of others; their singular apparatus of arms, bearing suckers, their strange forms, and the inky secretion with which they can darken the water, have at all times attracted popular attention. The great complexity of their structures, and the fact that in many points they stand quite at the head of the invertebrates of the sea, and approach most nearly to the elevation of the true fishes, have secured to them the attention of naturalists. Some of these animals have shelly internal supports, and one genus, that of the Argonauts, or Paper Nautili, has an external protective shell. Allied, though more distantly, to the Cuttle-fishes, are the true Nautili, represented in the modern sea principally by the Pearly Nautilus, though there are two other species, both of them very rare. The modern pearly nautilus (Fig. 65) may be regarded as a peculiar kind of cuttle-fish provided with a discoidal shell for protection, and also for floatage. The shell is divided into a number of chambers by partitions. Of these the animal inhabits the last and largest. The others are empty, and are connected with the body of the animal only by a pipe, or siphuncle, with membranous walls and filled with fluid. Thus provided, the nautilus, when in the water, has practically no weight, and canmove up or down in the sea with the greatest facility, using its sucker-bearing arms and horny beak to seize and devour the animals on which it preys. The buoyancy of the shell seems exactly adapted to the weight of the animal; and this proportion is kept up by the addition of new air-chambers as the body increases in size. In the modern seas this singular little group stands entirely isolated, and its individuals are so rare that it is difficult to procure perfect specimens for collections, though its mechanical structure and advantages for the struggle for existence seem of the highest order. But in the old world of past geological time the case was altogether different.
Squid (Loligo)
Pearly Nautilus.
Fig. 64.—Squid (Loligo).
Fig. 65.—Pearly Nautilus (Nautilus pompilius).
a, Mantle.b, Its dorsal fold.c, Hood.o, Eye.t, Tentacles.f, Funnel.g, Air chambers.h, Siphuncle.
The Nautiloid shell-fishes burst suddenly upon us in the beginning of the Siluro-Cambrian, or Lower Silurian, Barrande’s second fauna; and this applies to all the countries wherethey have been studied. In this formation alone about 450 species are known, and in the Silurian these increase to 1,200; and here the group culminates. It returns in the Devonian to about the same number with the Lower Silurian, diminishes in the Carboniferous to 350, and in the Mesozoic, where the Nautiloid forms are replaced by others of the type of the Ammonites, becomes largely reduced. In the Tertiary there are but nineteen species, and, as already stated, in the modern worldthree. These statements do not, however, represent the whole truth. In the Palæozoic, in addition to the genusNautilus, we have a great number of other genera, some with perfectly straight shells, likeOrthoceras(Fig. 66), othersbent (Cyrtoceras), others differing in the style of siphuncle, or aperture, or chambers (Endoceras,Gomphoceras,Lituites,Figs. 67 to 69), or inflated into sac-like forms (Ascoceras). There is, besides, the family of theGoniatidæ(Fig. 70), with the chambers thrown into angular folds and the siphuncle at the back. Further, some of the early forms, as the Orthoceratidæ,attain to gigantic dimensions, being six feet or more in length, and nearly a foot in diameter. Thus the idea that we should naturally form from the study of the Nautilus, that it represents a type suited for much more varied and important adaptations than those that we now see, is more than realised in those Palæozoic ages when these animals seem to have been the lords of the seas.
Orthoceras. Siluro-Cambrian.
Fig. 66.—Orthoceras. Siluro-Cambrian. The dotted line shows the position of the siphuncle.
Gomphoceras.
Fig. 67.—Gomphoceras.
Lituites.
Fig. 68.—Lituites.
Nautilus Avonensis.
Fig. 69.—Nautilus Avonensis(Dn.). Carboniferous.
a, Shell, reduced.b, Section, showing siphuncle.
Goniatites crenistria.
Fig. 70.—Goniatites crenistria(Philips). Carboniferous.
Ceratites nodosus
Fig. 71.—Ceratites nodosus(Schloth). Triassic.
When we leave the Palæozoic and enter the Mesozoic, though the Nautiloid shells still abound, we find them superseded, in great part, by a nobler form, that of theAmmonitidæ(Figs. 71, 72). These are remarkable for the ornate markings on the surfaces of their shells, and for the beautifully waved edges of the partitions (Fig. 72a), which, by giving a much more complete support to the sides of the shell, must have contributed greatly to the union of lightness and strength so important to the utility of the shell as a float. This type admits of all the same variety of straight, bent, and curled forms with the simpler Nautiloid type, and some of the species are of great size, Ammonites being known three feet or more in diameter. These animals, unknown in the Palæozoic, appear in numerous speciesin the Early Mesozoic, culminate in hundreds of beautiful species in the middle of that era, and disappear for ever at its close, leaving no modern successors. Many and beautiful species of Ammonites and their allies have been obtained from the Mesozoic rocks of British Columbia and other parts of the west coast of North America, perfectly representing this group as it occurs at the same period in Europe, and closely resembling the Mesozoic Ammonites of India. These animals have all perished, yet the Atlantic and the Pacific roll between, apparently with conditions as favourable for their comfortable existence as those of any previous time. They perished long ago, at thedawn of the Tertiary; yet the genus Nautilus, one of the oldest and least improved of the whole, survived, and still testifies to the wonderful contrivance embodied in these animals.
Ammonites Jason
Fig. 72.—Ammonites Jason(Reinecke). Jurassic.
Suture of Ammonites componens.
Fig.72a.—Suture ofAmmonites componens(Meek), of British Columbia. Showing the complicated folding of the edges of the chambers to give strength to the shell. Cretaceous.
Cretaceous Ammonitidae.
Fig. 73.—Cretaceous Ammonitidæ.
a,Baculites.b,Ancyloceras.c,Crioceras.d,Turrilites.
These are merely general considerations, but Barrande, in hisÉtudes Générales, goes much farther. He sums up all the known facts in the most elaborate manner, considering first the embryonic characters of the shell in the different genera, then their distribution in space and time, then all the different parts and characters of the shells in the different groups—the whole with reference to any possible derivation of the species; and he finds that all leads to the result that in every respect these shells seem to have been so introduced as to make any theory of evolution with respect to them altogether untenable. In his concluding sentence this greatest of Palæozoic palæontologistsaffirms that, “The theoretical evolution of the Cephalopods is, like that of the Trilobites, a mere figment of imagination, without any foundation in fact.”18
Belemnite.
Fig. 74.—Belemnite.—After Philips.
Belemnoteuthis antiquus.
Fig.74a.—Belemnoteuthis antiquus.Supposed to be a Belemnite, with soft parts preserved.—Jurassic.—After Mantell.
I have reserved no space to notice the geological history of the other and higher group of Cephalopods, including the true Cuttles and Squids. This is perhaps less to be regretted, as, from the absence of external shells, they are likely to be much less perfectly known as fossils. So far as known, they are vastly younger than the Nautiloids, for no examples whatever have been found in the Palæozoic. They appear abundantly in the Mesozoic, but are there represented principally by an extinct group of squids (Belemnites and their allies,Figs. 74, 74a), remarkable for the great and complicated development of their internal support, which has a chambered float as well as a solid sheath. This family becomes extinct at the close of theMesozoic, though the cuttles as a whole perhaps culminate in the modern.
Cambrian Trilobites.
Fig. 75.—Cambrian Trilobites.
a,Paradoxides.b,Dikellocephalus.c,Conocoryphe(head).d,Agnostus(head and tail).
The remarkable group of the Trilobites had precedence in order of time of the Nautiloid shell-fishes. No animal structures can well be more dissimilar than those of the two great groups of aquatic animals which popular speech confounds under the name of “shell-fishes.” Take a whelk and a crab, for example, and compare their general forms, the structure of their shells, and their organs of motion, and it is scarcely possible to imagine any two animals more unlike; and when we examine their anatomy in detail this difference does not diminish. They have, it is true, corresponding parts, and these parts serve similar uses, but in plan of structure they are wholly different. Yet both animals may live in the same pool, and may subsist on nearly the same food. If we attempt to find some common type which both resemble, we may trace the structure of the crab back to those of some of the marine worms with which it has some affinity, and those of the whelk to such creatures as theLingula, which are supposed to have a resemblance tothe worms. But still the two types, that of the Mollusk and the Articulate, are distinct even from their first appearance in the egg, nor have either any close affinities with the Protozoa, the Hydroids, or the Corals.
Transverse section of Calymene.
Fig. 76.—Transverse section ofCalymene. A Silurian Trilobite.—After Wolcott.
a, Dorsal shell.b, Visceral cavity.c, Legs.d, Epipodite—gill-cleaner or palp.e, Spiral gills.
Both types meet us in the Early Cambrian, but while the Mollusk is there represented only by low forms, the Articulate is then not only in the humble guise of the worm, but in the complex and highly organised form of the Trilobite (Figs. 28 and 75). What older phases they may have passed through we know not; but in the Lower Cambrian we have various forms of these animals, including some of the largest known as well as some of the smallest; some of the most complex in number of parts as well as some of the simplest. These animals, in short, seem to have appeared at once all over the world fully formed, and in a variety of generic and specific forms; and nothing short of a very large faith in the imperfection of the geological record can suffice to account for their evolution.
Burrows of Trilobite and of modern King-crab.
Fig.76a.—Burrows of Trilobite and of modern King-crab. The Trilobite burrow is known as Ruschinites, and has been supposed to be a sea-weed of the kind calledBilobites.
A Trilobite is a creature in whose structure the number threeis dominant. Seen from above, it presents three divisions from front to rear:—first, a cephalic shield or head-piece; secondly, a thorax, divided into several segments movable upon each other; and thirdly, a tail-piece or pygidium, which, when brought against the head by the rolling up of the body segments, effectually covers the lower parts. This lower portion was until lately little known; but the discoveries of Billings and of Wolcott have enabled us to restore the jaws under the head, the jointed legs and spiral gills under the thorax, and thus to complete the structure of the animal, and understand better its relations to modern crabs and shrimps (Fig. 76). Of these it certainly comes nearest to the King-crabs and Horseshoe-crabs,a somewhat limited group at present, and one which reaches back in geological time only to the Upper Silurian, when the Trilobites had perhaps already passed their culmination.
Constructed as above described, the Trilobite could swim, as is supposed, usually on its back or side. It could crawl on the bottom. Using its snout as a shovel, it could burrow like a modern King-crab (Fig. 76a); and when pressed by danger some species could roll themselves into balls and defy their enemies.
Silurian Trilobites.
Fig. 77.—Silurian Trilobites.
a,Isotelus.b,Homalonotus.c,Calymene.
This type of animal, entering on the stage in full force in the Older Cambrian, continues under many forms through the whole Palæozoic age, dying out finally in the Carboniferous.Figs. 77 and 78show a few of the forms of the Silurian, Devonian, and Carboniferous.
Contemporaneously with the dawn of the Trilobite group, appear some small shrimp-like forms (Fig. 28),19and others with bivalve shells (Fig. 79), which are closely allied to modern forms,20and, like theLingulæ, persist through the succeedingformations with little more than specific change—presenting in this a strange contrast to the Trilobites. While the latter were still flourishing, about the close of the Lower Silurian, a remarkable group of large and highly-developed creatures, allied to the Trilobites, but suited for rapid swimming rather than creeping, was introduced; and in the Upper Silurian andDevonian these creatures21attained to gigantic sizes, exceeding, probably, any modern Crustaceans, and were tyrants of the seas.Pterygotus anglicus(Fig. 80) is supposed to have attained the length of six feet. Yet these noble representatives of the Crustaceans became extinct in the Carboniferous. On the other hand, a few small king-crabs appear in the Upper Silurian, and this type still continues, and seems to culminate as to size in modern times; so diverse have been the fortunes of these various groups.