FOOTNOTES:

Ramsay Heatley Traquair.Arthur Smith Woodward.Karl A. Zittel.Charles R. Eastman.

Ramsay Heatley Traquair.

Arthur Smith Woodward.

Karl A. Zittel.

Charles R. Eastman.

Second Period.—Systematic Study of Fossil Fishes.—On the ground planted by Agassiz, many important works sprang up within the next decades. In England a vigorous school of palæichthyologists was soon flourishing. Many papers of Egerton date from this time, and the important work of Owen on the structure of fossil teeth and the often-quoted papers of Huxley in the "British Fossil Remains." Among other workers may be mentioned James Powrie, author of a number of papers upon Scottish Devonian fossils; the enthusiastic Hugh Miller, stone-mason and geologist; Montague Brown, Thomas Atthey, J. Young, and W. J. Barkas, students upon Coal Measure fishes; E. Ray Lankester, some of whose early papers deal with pteraspids; E. T. Newton, author of important works on chimæroids. The extensive works of J. W. Davis deal with fishes of many groups and many horizons. Mr. Davis, like Sir Philip Gray Egerton, was an amateur whose devotion did much to advance the study of fossil fishes. The dean of British palæichthyology is at present Dr. R. H. Traquair, of the Edinburgh Museum of Science and Arts. During four decades he has devoted himself to his studies with rare energy and success, author of a host of shorter papers and numerous memoirs and reports. Finally, and belonging to a younger generation of palæontologists, is to be named Arthur Smith Woodward, curator of vertebrate palæontology of the British Museum. Dr. Woodward has already contributed many scores of papers to palæichthyology, besides publishing a four-volume Catalogue of the Fossil Fishes of the British Museum, a compendial work whose value can only be appreciated adequately by specialists.

In the United States the study of fossil fishes was taken up by J. H. and W. C. Redfield, father and son, prior to the work of Agassiz, and there has been since that time an active school of American workers. Agassiz himself, however, is not to be included in this list, since his interest in extinct fishes became almost entirely unproductive during his life in America. Foremost among these workers was John Strong Newberry (1822-92), of Columbia College, whose publications deal with fishes of many horizons and whose work upon this continent is not unlike that of Agassiz in Europe. He was the author of many state reports, separate contributions, and two monographs, one upon the palæozoic fishes of North America, the other upon the Triassic fishes. Among the earlier palæontologists were Orestes H. St. John, a pupil of Agassiz at Harvard, and A. H. Worthen (1813-88), director of the Geological Survey of Illinois; also W. Gibbes and Joseph Leidy. The late E. D. Cope (1840-97) devoted a considerable portion of his labors to the study of extinct fishes. E. W. Claypole, of Buchtel College, is next to be mentioned as having produced noteworthy contributions to our knowledge of sharks, palæaspids, and arthrodires, as has also A. A. Wright, of Oberlin College. Among other workers may be mentioned O. P. Hay, of the American Museum; C. R. Eastman, of Harvard, author of important memoirs upon arthrodires and other forms; Alban Stewart, a student of Dr. S. W. Williston at Kansas University, and Bashford Dean. Among Canadian palæontologists G. F. Matthew deserves mention for his work on Cyathaspis, Principal Dawson for interesting references to Mesozoic fishes, and J. F. Whiteaves for his studies upon the Devonian fishes of Scaumenac Bay.

Belgian palæontologists have also been active in their study of fishes. Here we may refer to the work of Louis Dollo, of Brussels, of Max Lohest, of P. J. van Beneden, of L. G. de Koninck, of T. C. Winckler, and of R. Storms, the last of whom has done interesting work on Tertiary fishes.

Foremost among Russian palæichthyologists is to be named C. H. Pander, long-time Academician in St. Petersburg, whose elaborate studies of extinct lung-fishes, ostracophores, and crossopterygians published between 1856 and 1860 will long stand as models of careful work. We should also refer to the work of H. Asmuss and H. Trautschold, E. Eichwald and of Victor Rohon, the last named having published many important papers upon ostracophores during his residence in St. Petersburg.

German palæichthyologists include Otto Jaekel, of Berlin; O. M. Reis of the Oberbergamt, in Munich; A. von Koenen, of Göttingen; A. Wagner, E. Koken, and K. von Zittel. Among Austro-Hungarians are Anton Fritsch, author of theFauna der Gaskohleformations Boemens; Rudolf Kner, an active student of living fishes as well, as is also Franz Steindachner.

French palæichthyologists are represented by the veteran H. E. Sauvage, of Boulogne-sur-Mer, V. Thollière, M. Brongniart, and F. Priem. In Italy Francesco Bassani, of Naples, is the author of many important works dealing with Mesozoic and Tertiary forms; also was Baron Achille di Zigno. Robert Collett, of Bergen, and G. Lindström are worthy representatives of Scandinavia in kindred work.

Third Period.—Morphological Work on Fossil Fishes.—Among the writers who have dealt with the problems of the relationships of the Ostracophores as well asPalæospondylusand the Arthrodires may be named Traquair, Huxley, Newberry, Smith Woodward, Rohon, Eastman, and Dean; most recently William Patten. Upon the phylogeny of the sharks Traquair, A. Fritsch, Hasse, Cope, Brongniart, Jaekel, Reis, Eastman, and Dean. On Chimæroid morphology mention may be made of the papers of A. S. Woodward, Reis, Jaekel, Eastman, C. D. Walcott, and Dean. As to Dipnoan relationships the paper of Louis Dollo is easily of the first value; of especial interest, too, is the work of Eastman as to the early derivation of the Dipnoan dentition. In this regard a paper of Rohon is noteworthy, as is also that of Richard Semon on the development of the dentition of recent Neoceratodus, since it contains a number of references to extinct types. Interest notes on Dipnoan fin characters have been given by Traquair. In the morphology of Ganoids, the work of Traquair and A. S. Woodward takes easily the foremost rank. Other important works are those of Huxley, Cope, A. Fritsch, and Oliver P. Hay.

Anatomists.—Still more difficult of enumeration is the long list of those who have studied the anatomy of fishes usually in connection with the comparative anatomy or development of other animals. Pre-eminent among these are Karl Ernst von Baer, Cuvier, Geoffroy St. Hilaire, Louis Agassiz, Johannes Müller, Carl Vogt, Carl Gegenbaur, William Kitchen Parker, Francis M. Balfour, Thomas Henry Huxley, Meckel, H. Rathke, Richard Owen, Kowalevsky, H. Stannius, Joseph Hyrtl, Gill, Boulenger, and Bashford Dean. Other names of high authority are those of Wilhelm His, Kölliker, Bakker, Rosenthal, Gottsche, Miklucho-Macleay, Weber, Hasse, Retzius, Owsjannikow, H. Müller, Stieda, Marcusen, J. A. Ryder, E. A. Andrews, T. H. Morgan, G. B. Grassi, R. Semon, Howard Ayers, R. R. Wright, J. P. McMurrich, C. O. Whitman, A. C. Eyclesheimer, E. Pallis, Jacob Reighard, and J. B. Johnston.

Besides all this, there has risen, especially in the United States, Great Britain, Norway, and Canada and Australia, a vast literature of commercial fisheries, fish culture, and angling, the chief workers in which fields we may not here enumerate even by name.

FOOTNOTES:[148]For these paragraphs on the history of the study of fossil fishes the writer is indebted to the kind interest of Professor Bashford Dean.[149]Dr. Arthur Smith Woodward excepted.

[148]For these paragraphs on the history of the study of fossil fishes the writer is indebted to the kind interest of Professor Bashford Dean.

[149]Dr. Arthur Smith Woodward excepted.

Howto Secure Fishes.—In collecting fishes three things are vitally necessary—a keen eye, some skill in adapting means to ends, and some willingness to take pains in the preservation of material.

In coming into a new district the collector should try to preserve the first specimen of every species he sees. It may not come up again. He should watch carefully for specimens which look just a little different from their fellows, especially for those which are duller, less striking, or with lower fins. Many species have remained unnoticed through generations of collectors who have chosen the handsomest or most ornate specimens. In some groups with striking peculiarities, as the trunkfishes, practically all the species were known to Linnæus. No collector could pass them by. On the other hand, new gobies or blennies can be picked up almost every day in the lesser known parts of the world. For these overlooked forms—herrings, anchovies, sculpins, blennies, gobies, scorpion-fishes—the competent collector should be always on the watch. If any specimen looks different from the rest, take it at once and find out the reason why.

In most regions the chief dependence of the collector is on the markets and these should be watched most critically. By paying a little more for unusual, neglected, or useless fish, the supply of these will rise to the demand. The word passed along among the people of Onomichi in Japan, that "Ebisu the fish-god was in the village" and would pay more for okose (poison scorpion-fishes) and umiuma (sea-horses) than real fishes were worth soon brought (in 1900) all sorts of okose and umiuma into the market when they were formerly left neglected on the beach. Thus with a little ingenuity the markets in any country can be greatly extended.

The collector can, if he thinks best, use all kinds of fishing tackle for himself. In Japan he can use the "dabonawa" long lines, and secure the fishes which were otherwise dredged by theChallengerandAlbatross. If dredges or trawls are at his hand he can hire them and use them for scientific purposes. He should neglect no kind of bottom, no conditions of fish life which he can reach.

Especially important is the fauna of the tide-pools, neglected by almost all collectors. As the tide goes down, especially on rocky capes which project into the sea, myriads of little fishes will remain in the rock-pools, the algæ, and the clefts of rock. In regions like California, where the rocks are buried with kelp, blennies will lie in the kelp as quiescent as the branches of the algæ themselves until the flow of water returns.

A sharp three-tined fork will help in spearing them. The water in pools can be poisoned on the coast of Mexico with the milky juice of the "hava" tree, a tree which yields strychnine. In default of this, pools can be poisoned by chloride of lime, sulphate of copper, or, if small enough, by formaline. Of all poisons the commercial chloride of lime seems to be most effective. By such means the contents of the pool can be secured and the next tide carries away the poison. The water in pools can be bailed out, or, better, emptied by a siphon made of small garden-hose or rubber tubing. On rocky shores, dynamite can be used to advantage if the collector or his assistant dare risk it and if the laws of the country do not prevent.

Most effective in rock-pool work is the help of the small boy. In all lands the collector will do well to take him into his pay and confidence. Of the hundred or more new species of rock-pool fishes lately secured by the writer in Japan, fully two-thirds were obtained by the Japanese boys. Equally effective is the "muchacho" on the coasts of Mexico.

Masses of coral, sponges, tunicates, and other porous or hollow organisms often contain small fishes and should be carefully examined. On the coral reefs the breaking up of large masses is often most remunerative.

The importance of securing the young of pelagic fishes by tow-nets and otherwise cannot be too strongly emphasized.

How to Preserve Fishes.—Fishes must be permanently preserved in alcohol. Dried skins are far from satisfactory, except as a choice of difficulties in the case of large species.

Dr. Günther thus describes the process of skinning fishes:

"Scaly fishes are skinned thus: With a strong pair of scissors an incision is made along the median line of the abdomen from the foremost part of the throat, passing on one side of the base of the ventral and anal fins to the root of the caudal fin, the cut, being continued upward to the back of the tail close to the base of the caudal. The skin of one side of the fish is then severed with the scalpel from the underlying muscles to the median line of the back; the bones which support the dorsal and caudal are cut through, so that these fins remain attached to the skin. The removal of the skin of the opposite side is easy. More difficult is the preparation of the head and scapulary region. The two halves of the scapular arch which have been severed from each other by the first incision are pressed toward the right and left, and the spine is severed behind the head, so that now only the head and shoulder bones remain attached to the skin. These parts have to be cleaned from the inside, all soft parts, the branchial and hyoid apparatus, and all smaller bones being cut away with the scissors or scraped off with the scalpel. In many fishes which are provided with a characteristic dental apparatus in the pharynx (Labroids, Cyprinoids), the pharyngeal bones ought to be preserved and tied with a thread to their specimen. The skin being now prepared so far, its entire inner surface as well as the inner side of the head are rubbed with arsenical soap; cotton-wool or some other soft material is inserted into any cavities or hollows, and finally a thin layer of the same material is placed between the two flaps of the skin. The specimen is then dried under a slight weight to keep it from shrinking.

"The scales of some fishes, as for instance of many kinds of herrings, are so delicate and deciduous that the mere handling causes them to rub off easily. Such fishes may be covered with thin-paper (tissue paper is the best) which is allowed to dry on them before skinning. There is no need for removing the paper before the specimen has reached its destination.

"Scaleless fishes, as siluroids and sturgeons, are skinned inthe same manner, but the skin can be rolled up over the head; such skins can also be preserved in spirits, in which case the traveler may save to himself the trouble of cleaning the head.

"Some sharks are known to attain to a length of thirty feet, and some rays to a width of twenty feet. The preservation of such gigantic specimens is much to be recommended, and although the difficulties of preserving fishes increase with their size, the operation is facilitated, because the skins of all sharks and rays can easily be preserved in salt and strong brine. Sharks are skinned much in the same way as ordinary fishes. In rays an incision is made not only from the snout to the end of the fleshy part of the tail, but also a second across the widest part of the body. When the skin is removed from the fish, it is placed into a cask with strong brine mixed with alum, the head occupying the upper part of the cask; this is necessary, because this part is most likely to show signs of decomposition, and therefore most requires supervision. When the preserving fluid has become decidedly weaker from the extracted blood and water, it is thrown away and replaced by fresh brine. After a week's or fortnight's soaking the skin is taken out of the cask to allow the fluid to drain off; its inner side is covered with a thin layer of salt, and after being rolled up (the head being inside) it is packed in a cask the bottom of which is covered with salt; all the interstices and the top are likewise filled with salt. The cask must be perfectly water-tight."

Value of Formalin.—In the field it is much better to use formalin (formaldehyde) in preference to alcohol. This is an antiseptic fluid dissolved in water, and it at once arrests decay, leaving the specimen as though preserved in water. If left too long in formalin fishes swell, the bones are softened, and the specimens become brittle or even worthless. But for ordinary purposes (except use as skeleton) no harm arises from two or three months' saturation in formalin. The commercial formalin can be mixed with about twenty parts of water. On the whole it is better to have the solution too weak rather than too strong. Too much formalin makes the specimens stiff, swollen, and intractable, besides too soon destroying the color.

Formalin has the advantage, in collecting, of cheapness and of ease in transportation, as a single small bottle will makea large amount of the fluid. The specimens also require much less attention. An incision should be made in the (right) side of the abdomen to let in the fluid. The specimen can then be placed in formalin. When saturated, in the course of the day, it can be wrapped in a cloth, packed in an empty petroleum can, and at once shipped. The wide use of petroleum in all parts of the world is a great boon to the naturalist.

Before preservation, the fishes should be washed, to remove slime and dirt. They should have an incision to let the fluid into the body cavity and an injection with a syringe is a useful help to saturation, especially with large fishes. Even decaying fishes can be saved with formalin.

Records of Fishes.—The collector should mark localities most carefully with tin tags and note-book records if possible. He should, so far as possible, keep records of life colors, and water-color sketches are of great assistance in this matter. In spirits or formalin the life colors soon fade, although the pattern of marking is usually preserved or at least indicated. A mixture of formalin and alcohol is favorable to the preservation of markings.

In the museum all specimens should be removed at once from formalin to alcohol. No substitute for alcohol as a permanent preservative has been found. The spirits derived from wine, grain, or sugar is much preferable to the poisonous methyl or wood alcohol.

In placing specimens directly into alcohol, care should be taken not to crowd them too much. The fish yields water which dilutes the spirit. For the same reason, spirits too dilute are ineffective. On the other hand, delicate fishes put into very strong alcohol are likely to shrivel, a condition which may prevent an accurate study of their fins or other structures. It is usually necessary to change a fish from the first alcohol used as a bath into stronger alcohol in the course of a few days, the time depending on the closeness with which fishes are packed. In the tropics, fishes in alcohol often require attention within a few hours. In formalin there is much less difficulty with tropical fishes.

Fishes intended for skeletons should never be placed in formalin. A softening of the bones which prevents futureexact studies of the bones is sure to take place. Generally alcohol or other spirits (arrack, brandy, cognac, rum, sake "vino") can be tested with a match. If sufficiently concentrated to be ignited, they can be safely used for preservation of fishes. The best test is that of the hydrometer. Spirits for permanent use should show on the hydrometer 40 to 60 above proof. Decaying specimens show it by color and smell and the collector should be alive to their condition. One rotting fish may endanger many others. With alcohol it is necessary to take especial pains to ensure immediate saturation. Deep cuts should be made into the muscles of large fishes as well as into the body cavity. Sometimes a small distilling apparatus is useful to redistil impure or dilute alcohol. The use of formalin avoids this necessity.

Small fishes should not be packed with large ones; small bottles are very desirable for their preservation. All spinous or scaly fishes should be so wrapped in cotton muslin as to prevent all friction.

Eternal Vigilance.—The methods of treating individual groups of fishes and of handling them under different climatic and other conditions are matters to be learned by experience. Eternal vigilance is the price of a good collection, as it is said to be of some other good things. Mechanical collecting—picking up the thing got without effort and putting it in alcohol without further thought—rarely serves any useful end in science. The best collectors are usually the best naturalists. The collections made by the men who are to study them and who are competent to do so are the ones which most help the progress of ichthyology. The student of a group of fishes misses half the collection teaches if he has made no part of it himself.

TheGeological Distribution of Fishes.—The oldest unquestioned remains of fishes have been very recently made known by Mr. Charles D. Walcott, from rocks of the Trenton period in the Ordovician or Lower Silurian. These are from Cañon City in Colorado. Among these is certainly a small Ostracophore (Asteraspis desideratus). With it are fragments (Dictyorhabdus) thought to be the back-bone of a Chimæra, but more likely, in Dean's view, the axis of a cephalopod, besides bony, wrinkled scales, referred with doubt to a supposed Crossopterygian genus calledEriptychius. This renders certain the existence ofOstracophoresat this early period, but their association withChimærasand Crossopterygians is questionable. Primitive sharks may have existed in Ordovician times, but thus far no trace of them has been found.

Fig. 246.—Fragment of Sandstone from Ordovician deposits, Cañon City, Colo., showing fragments of scales, etc., the earliest known traces of vertebrates. (From nature.)

The fish-remains next in age in America are from the Bloomfield sandstone in Pennsylvania of the Onondaga period in theupper Silurian. The earliest in Europe are found in the Ludlow shales, both of these localities being in or near the horizon of the Niagara rocks, in the Upper Silurian Age.

It is, however, certain that these Lower Silurian remains do not represent the beginning of fish-life. ProbablyOstracophores, andArthrodires, with perhaps Crossopterygians and Dipnoans, existed at an earlier period, together perhaps with unarmed, limbless forms without jaws, of which no trace whatever has been left.

Fig. 247.—Fossil fish remains from Ordovician rocks, Cañon City, Colo. (After Walcott.)a.Scale ofEriptychius americanusWalcott. FamilyHoloptychiidæ?b.Dermal plate ofAsteraspis desideratusWalcott. FamilyAsterolepidæ.c.Dictyorhabdus priscusWalcott, a fragment of uncertain nature, thought to be a chordal sheath of a Chimæra, but probably part of a Cephalopod (Dean).Chimæridæ?

The Earliest Sharks.—The first actual trace of sharks is found in the Upper Silurian in the form of fin-spines (Onchus), thought to belong to primitive sharks, perhaps Acanthodeans possibly to Ostracophores. With these are numerous bony shields of the mailed Ostracophores, and somewhat later those of the more highly specialized Arthrodires. Later appear the teeth ofCochliodontidæ, with Chimæras, a few Dipnoans, and Crossopterygians.

Devonian Fishes.—In the Devonian Age theOstracophoresincrease in size and abundance, disappearing with the beginningof the Carboniferous. The Arthrodires also increase greatly in variety and in size, reaching their culmination in the Devonian, but not disappearing entirely until well in the Carboniferous. These two groups (often united by geologists under the older name Placoderms) together with sharks and a few Chimæras made up almost exclusively the rich fish-fauna of Devonian times. The sharks were chiefly Acanthodean and Psammodont, as far as our records show. The supposed more primitive type ofCladoselacheis not known to appear before the latter part of the Devonian Age, whilePleuracanthusandCladodus, sometimes regarded as still more primitive, are as yet found only in the Carboniferous. It is clear that the records of early shark life are still incomplete, whatever view we may adopt as to the relative rank of the different forms. Chimæroids occur in the Devonian, and with them a considerable variety of Crossopterygians and Dipnoans. The true fishes appear also in the Devonian in the guise of the Ganoid ancestors and relatives ofPalæoniscum, all with diamond-shaped enameled scales. In the Devonian, too, we find the minute creaturePalæospondylus, our ignorance of which is concealed under the nameCycliæ.

Carboniferous Fishes.—In the Carboniferous Age the sharks increase in number and variety, the Ostracophores disappear, and the Arthrodires follow them soon after, the last being recorded from the Permian. Other forms of Dipnoans, Crossopterygians, and some Ganoids now appear giving the fauna a somewhat more modern aspect. TheAcanthodeiand theIchthyotomipass away with the Permian, the latest period of the Carboniferous Age.

Fig. 248.—Dipterus valenciennesiAgassiz, a Dipnoan. (After Dean, from Woodward.)

Mesozoic Fishes.—In the Triassic period which follows the Permian, the earliest types of Ganoids give place to forms approaching the garpike and sturgeon. The Crossopterygians rapidly decline. The Dipnoans are less varied and fewer in number; the primitive sharks, with the exception of certain Cestracionts, all disappear, only the family ofOrodontidæremaining. Here are found the first true bony fishes, doubtless derived from Ganoid stock, the allies and predecessors of the great group of herrings. Herring-like forms become more numerous in the Jurassic, and with them appear other forms which give the fish-fauna of this period something of a modern appearance. In the Jurassic the sharks become divided into several groups,Notidani, Scyllioid sharks, Lamnoid sharks, angel-fishes, skates, and finally Carcharioid sharks being now well differentiated. Chimæras are still numerous. TheAcanthodeihave passed away, as well as the mailed Ostrachopores and Arthrodires. The Dipnoans and Crossopterygians are few. The early Ganoids have given place to more modern types, still in great abundance and variety. This condition continues in the Cretaceous period. Here the rays and modern sharks increase in number, the Ganoids hold their own, and the other groups of soft-rayed fishes, as the smelts, the lantern-fishes, the pikes, the flying-fishes, the berycoids and the mackerels join the group of herring-like forms which represent the modern bony fishes. In the Cretaceous appear the first spiny-rayed fishes, derived probably from herring-like forms. These are allies or ancestors of the living genusBeryx.

Fig. 249.—Hoplopteryx lewesiensis(Mantell), restored. English Cretaceous. FamilyBerycidæ. (After Woodward.)

Dr. Woodward observes:

"As soon as fishes with a completely osseous endoskeleton began to predominate at the dawn of the Cretaceous period, specializations of an entirely new kind were rapidly acquired. Until this time the skull of the Actinopterygii had always been remarkably uniform in type. The otic region of the cranium often remained incompletely ossified and was never prominent or projecting beyond the roof bones; the supraoccipital bone was always small and covered with the superficial plates; the maxilla invariably formed the greater part of the upper jaw; the cheek-plates were large and usually thick; while none of the head or opercular bones were provided with spines or ridges. The pelvic fins always retained their primitive remote situation, and the fin-rays never became spines. During the Cretaceous period the majority of the bony fishes began to exhibit modifications in all these characters, and the changes occurred so rapidly that by the dawn of the Eocene period the diversity observable in the dominant fish-fauna was much greater than it had ever been before. At this remote period, indeed, nearly all the great groups of bony fishes, as represented in the existing world, were already differentiated, and their subsequent modifications have been quite of a minor character."

Fig. 250.—A living Berycoid fish,Paratrachichthys prosthemiusJordan & Fowler. Misaki, Japan. FamilyBerycidæ.

Fig. 251.—Flying-fish,Cypsilurus heterurus(Rafinesque). FamilyExocætidæWoods Hole, Mass.

Fig. 252.—The Schoolmaster Snapper, a Perch-like fish. FamilyLutianidæ. Key West.

Tertiary Fishes.—With the Eocene or first period of the Tertiary great changes have taken place. The early families of bony fishes nearly all disappear. The herring, pike, smelt, salmon, flying-fish, and berycoids remain, and a multitude of other forms seem to spring into sudden existence. Among these are the globefishes, the trigger-fishes, the catfishes, the eels, the morays, the butterfly-fishes, the porgies, the perch, the bass, the pipefishes, the trumpet-fishes, the mackerels, and the John-dories, with the sculpins, the anglers, the flounders, the blennies, and the cods. That all these groups, generalized and specialized, arose at once is impossible, although all seemto date from the Eocene times. Doubtless each of them had its origin at an earlier period, and the simultaneous appearance is related to the fact of the thorough study of the Eocene shales, which have in numerous localities (London, Monte Bolca, Licata, Mount Lebanon, Green River) been especially favorable for the preservation of these forms. Practically fossil fishes have been thoroughly studied as yet only in a very few parts of the earth. The rocks of Scotland, England, Germany, Italy, Switzerland, Syria, Ohio, and Wyoming have furnished the great bulk of all the fish remains in existence. In some regions perhaps collections will be made which will give us a more just conception of the origin of the different groups of bony fishes. We can now only say with certainty that the modern families were largely existent in the Eocene, that they sprang from ganoid stock found in the Triassic and Jurassic, that several of them were represented in the Cretaceous also, that the Berycoids were earliest of the spiny-rayed fishes, and forms allied to herring the earliest of the soft-rayed forms. Few modern families arose before the Cretaceous. Few of the modern genera go back to the Eocene, many of them arose in the Miocene, and few species have come down to us from rocks older than the end of the Pliocene. The general modern type of the fish-faunas being determined in the latter Eocene and the Miocene, the changes which bring us to recent times have largely concerned the abundance and variety of the individual species. From geological distribution we have arising the varied problems of geographical distribution and the still more complex conditions on which depend the extinction of species and of types.

Fig. 253.—Decurrent Flounder,Pleuronichthys decurrensJordan & Gilbert. San Francisco.

Factors of Extinction.—These factors of extinction have been recently formulated as follows by Professor Herbert Osborn. He considers the process of extinction as of five different types:

"(1) That extinction which comes from modification or progressive evolution, a relegation to the past as a result of the transmutation into more advanced forms. (2) Extinction from changes of physical environment which outrun the powers of adaptation. (3) The extinction which results from competition. (4) The extinction which results from extreme specialization and limitation to special conditions the loss of which means extinction. (5) Extinction as a result of exhaustion."

Fossilization of a Fish.—When a fish dies he leaves no friends. His body is at once attacked by hundreds of creatures ranging from the one-celled protozoa and bacteria to individuals of his own species. His flesh is devoured, his bones are scattered, the gelatinous substance in them decays, and the phosphate of lime is in time dissolved in the water. For this reason few fishes of the millions which die each year leave any trace for future preservation. At the most a few teeth, a fin-spine, or a bone buried in the clay might remain intact or in such condition as to be recognized.

But now and then it happens that a dead fish may fall in more fortunate conditions. On a sea bottom of fine clay the bones, or even the whole body, may be buried in such a way as to be sealed up and protected from total decomposition. The flesh will usually disappear and leave no mark or at the most a mere cast of its surface. But the hard parts, even the muscles may persist, and now and then they do persist, the salts of lime unchanged or else silicified or subjected to some other form of chemical substitution. Only the scales, the teeth, the bones, the spines, and the fin-rays can be preserved in the rocks of sea orlake bottom. In a few localities, as near Green River in Wyoming, Monte Bolca, near Verona, and Mount Lebanon in Syria, the London clays, with certain quarries in Scotland and lithographic stones in Germany, many skeletons of fishes have been found pressed flat in layers of very fine rock, their structures traced as delicately as if actually drawn on the smooth stone. Fragments preserved in ruder fashion abound in the clays and even the sandstones of the earliest geologic ages. In most cases, however, fossil fishes are known from detached and scattered fragments, many of them, especially of the sharks, by the teeth alone. Fishes have occurred in all ages from the Silurian to the present time and probably the very first lived long before the Silurian.

The Earliest Fishes.—No one can say what the earliest fishes were like, nor do we know what was their real relation to the worm-like forms among which men have sought their presumable ancestors, nor to the Tunicates and other chordate forms, not fish-like, but still degenerate relatives of the primeval fish.

From analogy we may suppose that the first fishes which ever were bore some resemblance to the lancelet, for that is a fish-like creature with every structure reduced to the lowest terms. But as the lancelet has no hard parts, no bones, nor teeth, nor scales, nor fins, no traces of its kind are found among the fossils. If the primitive fish was like it in important respects, all record of this has probably vanished from the earth.

The Cyclostomes.—The next group of living fishes, the Cyclostomes, including the hagfishes and lampreys,—fishes with small skull and brain but without limbs or jaws,—stands at a great distance above the lancelet in complexity of structure, and equally far from the true fishes in its primitive simplicity. In fact the lamprey is farther from the true fish in structure than a perch is from an eagle. Yet for all that it may be an offshoot from the primitive line of fish descent. There is not much in the structure of the lamprey which may be preserved in the rocks. But the cartilaginous skull, the back-bone, fins, and teeth might leave their traces in soft clay or lithographic stone. But it is certain that they have not done so in any rocks yet explored, and it may be that the few existing lampreys owe their form and structure to a process of degradation from a more complex and more fish-like ancestry. The supposedlamprey fossil of the Devonian of Scotland,Palæospondylus, has little in common with the true lampreys.

The Ostracophores.—Besides the lampreys the Devonian seas swarmed with mysterious creatures covered with an armor of plate, fish-like in some regards, but limbless, without true jaws and very different from the true fishes of to-day. These are called Ostracophori, and some have regarded them as mailed lampreys, but they are more likely to be a degenerate or eccentric offshoot from the sharks, as highly modified or specialized lampreys, a side offshoot which has left no descendants among recent forms. Recently Professor Patten has insisted that the resemblance of their head-plates to those of the horseshoe crab (Limulus) is indicative of real affinity.

Among these forms in mail-armor are some in which the jointed and movable angles of the head suggest the pectoral spines of some catfishes. But in spite of its resemblance to a fin, the spine inPterichthyodesis an outgrowth of the ossified skin and has no more homology with the spines of fishes than the mailed plates have with the bones of a fish's cranium. In none of these fishes has any trace of an internal skeleton been found. It must have retained its primitive gelatinous character. There are, however, some traces of eyes, and the mucous channels of the lateral line indicate that these creatures possessed some other special senses.

Fig. 254.—An Ostracophore,Cephalaspis lyelliAgassiz, restored. Devonian. (After Agassiz, per Dean.)

Whatever the Ostracophores may be, they should not be included within the much-abused termGanoidei, a word which was once used in the widest fashion for all sorts of mailed fishes, but little by little restricted to the hard-scaled relatives and ancestors of the garpike of to-day.

The Arthrodires.—Dimly seen in the vast darkness of Paleozoic time are the huge creatures known as Arthrodires. These are mailed and helmeted fishes, limbless so far as we know,but with sharp, notched, turtle-like jaws quite different from those of the fish or those of any animal alive to-day. These creatures appear in Silurian rocks and are especially abundant in the fossil beds of Ohio, where Newberry, Claypole, Eastman, Dean and others have patiently studied the broken fragments of their armor. Most of them have a great casque on the head with a shield at the neck and a movable joint connecting the two. Among them was almost every variation in size and form.

Fig. 255.—An Arthrodire,Dinichthys intermediusNewberry, restored. Devonian, Ohio. (Family after Dean.)

These creatures have been often called ganoids, but with the true ganoids like the garpike they have seemingly nothing in common. They are also different from the Ostracophores. To regard them with Woodward as derived from ancestral Dipnoans is to give a possible guess as to their origin, and a very unsatisfactory guess at that. In any event these have all passed away in competition with the scaly fishes and sharks of later evolution, and it seems certain that they, like the mailed Ostracophores, have left no descendants.

The Sharks.—Next after the lampreys, but a long way after them in structure, come the sharks. With the sharks appear for the first time true limbs and the lower jaw. The upper jaw is, however, formed from the palate, and the shoulder-girdle is attached behind the skull. "Little is known," says Professor Dean, "of the primitive stem of the sharks, and even the lines of descent of the different members of the group can only be generally suggested. The development of recent forms has yielded few results of undoubted value to the phylogenist. It would appear as if paleontology alone could solve the puzzles of their descent."

Of the very earliest sharks in the Upper Silurian Age the remains are too scanty to prove much save that there were sharks in abundance and variety. Spines, teeth, fragments of shagreen, show that in some regards these forms were highly specialized. In the Carboniferous Age the sharks became highly varied and extensively specialized. Of the Paleozoic types, however, all but a single family seems to have died out, leaving Cestraciontes only in the Permian and Triassic. From these the modern sharks one and all may very likely have descended.

Origin of the Sharks.—Perhaps the sharks are developed from the still more primitive shark imagined as without limbs and with the teeth slowly formed from modification of the ordinary shagreen prickles. In determining the earliest among the several primitive types of shark actually known we are stopped by an undetermined question of theory. What is the origin of paired limbs? Are these formed, like the unpaired fins, from the breaking up of a continuous fold of skin, in accordance with the view of Balfour and others? Or is the primitive limb, as supposed by Gegenbaur, a modification of the bony gill-arch? Or again, as supposed by Kerr, is it a modification of the hard axis of an external gill?

If we adopt the views of Gegenbaur or Kerr, the earliest type of limb is the jointedarchipterygium, a series of consecutive rounded cartilaginous elements with a fringe of rays along its length. Sharks possessing this form of limb (Ichthyotomi) appear in the Carboniferous rocks, but are not known earlier. It may be that from these the Dipnoans, on the one hand, may be descended and, on the other, the true sharks and the Chimæras; but there is no certainty that the jointed arm or archipterygium of the Dipnoans is derived from the similar pectoral fin of theIchthyotomi.

On the other hand, if we regard the paired fins as parts of a lateral fold of skin, we find primitive sharks to bear out our conclusions. InCladoselacheof the Upper Devonian, the pectoral and the ventral fins are long and low, and arranged just as they might be if Balfour's theory were true.Acanthoessus, with a spine in each paired fin and no other rays, might be a specialization of this type or fin, andClimatius, with rows of spines in place of pectorals and ventrals, might be held tobear out the same idea. In all these the tail is less primitive than in theIchthyotomi. On the other hand, the vent inCladoselacheis thought by Dean to have been near the end of the tail. If this is the case, it should indicate a very primitive character. On the whole, though there is much to be said in favor of the primitive nature of theIchthyotomi(Pleuracanthus) with the tapering tail and jointed pectoral fin of a dipnoan, and other traits of a shark, yet, on the whole,Cladoselacheis probably nearer the origin of the shark-like forms.

The relatively primitive sharks calledNotidanihave the weakly ossified vertebræ joined together in pairs and there are six or seven gill-openings. This group has persisted to our day, the frilled shark (Chlamydoselachus) and the generaHexanchusandHeptranchiasstill showing its archaic characters.

Here the sharks diverge into two groups, the one with the vertebræ better developed and its calcareous matter arranged star-fashion. This forms Hasse's group ofAsterospondyli, the typical sharks. The earliest forms (Orodontidæ,Heterodontidæ) approach theNotidani, and so far as geological records go, precede all the other modern sharks. One such ancient type,Heterodontus, including the bullhead shark, and the Port Jackson shark, still persists. The others diverge to form the three chief groups of the cat-sharks (Scyliorhinus, etc.), the mackerel-sharks (Lamna, etc.), and the true sharks (Carcharhias, etc.).

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