CHAPTER III

ISOSPONDYLI

=THE Subclass Teleostei, or Bony Fishes.=—The fishes which still remain for discussion constitute the great subclass or series of _Teleostei_ (τελεός, true; οστέον, bone), or bony fishes. They lack wholly or partly the Ganoid traits, or show them only in the embryo. The tail is slightly, if at all, heterocercal; the actinosts of the pectoral fins are few and large, rarely over five in number, except among the eels; the fulcra disappear; the air-bladder is no longer cellular, except in very rare cases, nor does it assist in respiration. The optic nerves are separate, one running to each eye without crossing; the skeleton is almost entirely bony, the notochord usually disappearing entirely with age; the valves in the arterial bulb are reduced in number, and the spiral valve of the intestines disappears. Traces of each of the Ganoid traits may persist somewhere in some group, but as a whole we see a distinct specialization and a distinct movement toward the fish type, with the loss of characters distinctive of sharks, Dipnoans, and Ganoids. In a general way the skeleton of all Teleosts corresponds with that of the striped bass (see Figs. 22, 23, Vol. I), and the visceral anatomy is in all cases sufficiently like that of the sunfish (Fig. 16, Vol. I).

The mesocoracoid or precoracoid arch, found in all Ganoids, persists in the less specialized types of bony fishes, although no trace of it is found in the perch-like forms. With all this, there is developed among the bony fishes an infinite variety in details of structure. For this reason the _Teleostei_ must be broken into many orders, and these orders are very different in value and in degrees of distinctness, the various groups being joined by numerous and puzzling intergradations.

=Order Isospondyli.=—Of the various subordinate groups of bony fishes, there can be no question as to which is most primitive in structure, or as to which stands nearest the orders of Ganoids. Earliest of the bony fishes in geological time is the order of _Isospondyli_ (ἴσος, equal; σπόνδυλος, vertebra), containing the allies, recent or fossil, of the herring and the trout. This order contains those soft-rayed fishes in which the ventral fins are abdominal, a mesocoracoid or precoracoid arch is developed, and the anterior vertebræ are unmodified and essentially similar to the others. The orbitosphenoid is present in all typical forms. In certain forms of doubtful affinity (_Iniomi_) the mesocoracoid is wanting or lost in degeneration. Through the _Isospondyli_ all the families of fishes yet to be considered are apparently descended, their ancestors being Ganoid fishes and, still farther back, the Crossopterygians.

Woodward gives this definition of the _Isospondyli_: "Notochord varying in persistence, the vertebral centra usually complete, but none coalesced; tail homocercal, but hæmal supports not much expanded or fused. Symplectic bone present, mandible simple, each dentary consisting only of two elements (dentary and articulo-angular), with rare rudiments of a splenoid on the inner side. Pectoral arch suspended from the cranium; precoracoid (mesocoracoid) arch present; infraclavicular plates wanting. Pelvic (ventral) fins abdominal. Scales ganoid only in the less specialized families. In the living forms air-bladder connected with the œsophagus in the adult; optic nerves decussating (without chiasma), and intestine either wanting spiral valve or with an incomplete representative of it."

=The Classification of the Bony Fishes.=—The classification of fishes has been greatly complicated by the variety of names applied to groups which are substantially but not quite identical one with another. The difference in these schemes of classification lies in the point of view. In all cases a single character must be brought to the front; such characters never stand quite alone, and to lay emphasis on another character is to make an alteration large or small in the name or in the boundaries of a class or order. Thus the _Ostariophysi_ with the _Isospondyli_, _Haplomi_, and a few minor groups make up the great division of the _Abdominales_. These are fishes in which the ventral fins are abdominal, that is, inserted backward, so that the pelvis is free from the clavicle, the two sets of limbs being attached to different parts of the skeleton. Most of the abdominal fishes are also soft-rayed fishes, that is, without consecutive spines in the dorsal and anal fins, and they show a number of other archaic peculiarities. The Malacopterygians (μαλακός, soft; πτερύξ, fin) of Cuvier therefore correspond very nearly to the _Abdominales_. But they are not quite the same, as the spiny-rayed barracudas and mullets have abdominal ventrals, and many unquestioned thoracic or jugular fishes, as the sea-snails and brotulids, have lost, through degeneration, all of their fin-spines.

In nearly but not quite all of the Abdominal fishes the slender tube connecting the air-bladder with the œsophagus persists through life. This character defines Müller's order of _Physostomi_ (φυσός, bladder; στόμα, mouth), as opposed to his _Physoclysti_ (φυσός, bladder; κλεῖστός, closed), in which this tube is present in the embryo or larva only. Thus the _Thoracices_ and _Jugulares_, or fishes having the ventrals thoracic or jugular, together correspond almost exactly to the Acanthopterygians, (ακανθα, spine; πτερύξ, fin), or spiny-rayed fishes of Cuvier, or to the _Physoclysti_ of Müller. The Malacopterygians, the _Abdominales_, and the _Physostomi_ are in the same way practically identical groups. As the spiny-rayed fishes have mostly ctenoid scales, and the soft-rayed fishes cycloid scales, the _Physostomi_ correspond roughly to Agassiz's _Cycloidei_, and the _Physoclysti_ to his _Ctenoidei_.

But in none of these cases is the correspondence perfectly exact, and in any system of classification we must choose characters for primary divisions so ancient and therefore so permanent as to leave no room for exceptions. The extraordinary difficulty of doing this, with the presence of most puzzling intergradations, has led Dr. Gill to suggest that the great body of bony fishes, soft-rayed and spiny-rayed, abdominal, thoracic, and jugular alike, be placed in a single great order which he calls _Teleocephali_ (τελεός, perfect; κεφαλή, head). The aberrant forms with defective skull and membrane-bones he would separate as minor offshoots from this great mass with the name of separate orders. But while the divisions of _Teleocephali_ are not strongly differentiated, their distinctive characters are real, ancient, and important, while those of the aberrant groups, called orders by Gill (as _Plectognathi_, _Pediculati_, _Hemibranchii_), are relatively modern and superficial, which is one reason why they are more easily defined. There seems to us no special advantage in the retention of a central order _Teleocephali_, from which the divergent branches are separated as distinct orders.

While our knowledge of the osteology and embryology of most of the families of fishes is very incomplete, it is evident that the relationships of the groups cannot be shown in any linear series or by any conceivable arrangement of orders and suborders. The living teleost fishes have sprung from many lines of descent, their relationships are extremely diverse, and their differences are of every possible degree of value. The ordinary schemes have magnified the value of a few common characters, at the same time neglecting other differences of equal value. No system of arrangement which throws these fishes into large groups can ever be definite or permanent.

=Relationships of Isospondyli.=—For our purposes we may divide the physostomous fishes as understood by Müller into several orders, the most primitive, the most generalized, and economically the most important being the order of _Isospondyli_. This order contains those bony fishes which have the anterior vertebræ unaltered (as distinguished from the _Ostariophysi_), the skull relatively complete, or at least not eel-like, the mesocoracoid typically developed, but atrophied in deep-sea forms and finally lost, the orbitosphenoid present. In all the species the ventral fins are abdominal and normally composed of more than six rays; the air-duct is developed. The scales are chiefly cycloid and the fins are without true spines. In many ways the order is more primitive than _Nematognathi_, _Plectospondyli_, or _Apodes_. It is certain that it began earlier in geological time than any of these. On the other hand, the _Isospondyli_ are closely connected through the _Berycoidei_ with the highly specialized fishes. The continuity of the natural series is therefore interrupted by the interposition of the side branches of Ostariophysans and eels before considering the _Haplomi_ and the other transitional forms. The forms called _Iniomi_, which lack the mesocoracoid and the orbitosphenoid, have been lately transferred to the _Haplomi_ by Boulenger. This arrangement is probably a step in advance.

Ganoid traits are present in certain families of _Isospondyli_. Among these are the gular plate (found in _Amia_ and the _Elopidæ_), doubtless derived from the similar structure in earlier Ganoids; additional valves in the arterial bulb in the cellular air-bladder of _Notopterus_ and _Osteoglossum_, the spiral intestinal valve in _Chirocentridæ_, and the ganoid scales of the extinct _Leptolepidæ_.

=The Clupeoidea.=—The _Isospondyli_ are divisible into numerous families, which may be grouped roughly under three subdivisions, _Clupeoidea_, the herring-like forms; the _Salmonoidea_, the trout-like forms; and the _Iniomi_, or lantern-fishes, and their allies. The last-named group should probably be removed from the order of _Isospondyli_. In the _Clupeoidea_, the allies of the great family of the herring, the shoulder-girdle is normally developed, retaining the mesocoracoid arch on its inner edge, and through the post-temporal is articulated above with the cranium. The fishes in this group lack the adipose fin which is characteristic of most of the higher or salmon-like families.

=The Leptolepidæ.=—Most primitive of the _Isospondyli_ is the extinct family of _Leptolepidæ_, closely allied to the Ganoid families of _Pholidophoridæ_ and _Oligopleuridæ_. It is composed of graceful, herring-like fishes, with the bones of the head thin but covered with enamel, and the scales thin but firm and enameled on their free portion. There are no fulcra and there is no lateral line. The vertebræ are well developed, but always pierced by the notochord. The genera are _Lycoptera_, _Leptolepis_, _Æthalion_, and _Thrissops_. In _Lycoptera_ of the Jurassic of China the vertebral centra are feebly developed, and the dorsal fin short and posterior. In _Leptolepis_ the anal is short and placed behind the dorsal. There are many species, mostly from the Triassic and lithographic shales of Europe, one being found in the Cretaceous. _Leptolepis coryphænoides_ and _Leptolepis dubius_ are among the more common species. _Æthalion_ (_knorri_) differs in the form of the jaws. In _Thrissops_ the anal fin is long and opposite the dorsal. _Thrissops salmonea_ is found in the lithographic stone; _Thrissops exigua_ in the Cretaceous. In all these early forms there is a hard casque over the brain-cavity, as in the living types, _Amia_ and _Osteoglossum_.

=The Elopidæ.=—The family of _Elopidæ_ contains large fishes herring-like in form and structure, but having a flat membrane-bone or gular plate between the branches of the lower jaw, as in the Ganoid genus _Amia_. The living species are few, abounding in the tropical seas, important for their size and numbers, though not valued as food-fishes save to those who, like the Hawaiians and Japanese, eat fishes raw. These people prefer for that purpose the white-meated or soft-fleshed forms like _Elops_ or _Scarus_ to those which yield a better flavor when cooked.

The ten-pounder (_Elops saurus_), pike-like in form but with very weak teeth, is found in tropical America. _Elops machnata_, the jack mariddle, the awaawa of the Hawaiians, abounding in the Pacific, is scarcely if at all different.

The tarpon, called also grande écaille, silver-king, and sable (_Tarpon atlanticus_), is a favorite game-fish along the coasts of Florida and Carolina. It takes the hook with great spirit, and as it reaches a length of six feet or more it affords much excitement to the successful angler. The very large scales are much used in ornamental work.

A similar species of smaller size, also with the last ray of the dorsal very much produced, is _Megalops cyprinoides_ of the East Indies. Other species occur in the South Seas.

Numerous fossil genera related to _Elops_ are found in the Cretaceous and Tertiary rocks. _Holcolepis lewesiensis_ (wrongly called _Osmeroides_) is the best-known European species. Numerous species are referred to _Elopopsis_. _Megalops prisca_ and species of _Elops_ also occur in the London Eocene.

In all these the large parietals meet along the median line of the skull. In the closely related family of _Spaniodontidæ_ the parietals are small and do not meet. All the species of this group, united by Woodward with the _Elopidæ_, are extinct. These fishes preceded the _Elopidæ_ in the Cretaceous period. Leading genera are _Thrissopater_ and _Spaniodon_, the latter armed with large teeth. _Spaniodon blondeli_ is from the Cretaceous of Mount Lebanon. Many other species are found in the European and American Cretaceous rocks, but are known from imperfect specimens only.

_Sardinius_, an American Cretaceous fossil herring, may stand near _Spaniodon_. _Rhacolepis buccalis_ and _Notelops brama_ are found in Brazil, beautifully preserved in concretions of calcareous mud supposed to be of Cretaceous age.

The extinct family of _Pachyrhizodontidæ_ is perhaps allied to the _Elopidæ_. Numerous species of _Pachyrhizodus_ are found in the Cretaceous of southern England and of Kansas.

=The Albulidæ.=—The _Albulidæ_, or lady-fishes, characterized by the blunt and rounded teeth, are found in most warm seas. _Albula vulpes_ is a brilliantly silvery fish, little valued as food. The metamorphosis (see Fig. 112, Vol. I) which the larva undergoes is very remarkable. It is probably, however, more or less typical of the changes which take place with soft-rayed fishes generally, though more strongly marked in _Albula_ and in certain eels than in most related forms. Fossils allied to _Albula_, _Albula oweni_, _Chanoides macropomus_, are found in the Eocene of Europe; _Syntegmodus altus_ in the Cretaceous of Kansas. In _Chanoides_, the most primitive genus, the teeth are much fewer than in _Albula_. _Plethodus_ and _Thryptodus_, with peculiar dental plates on the roof and floor of the mouth, probably constitute a distinct family, _Thryptodontidæ_. The species are found in European and American rocks, but are known from imperfect specimens only.

=The Chanidæ.=—The _Chanidæ_, or milkfishes, constitute another small archaic type, found in the tropical Pacific. They are large, brilliantly silvery, toothless fishes, looking like enormous dace, swift in the water, and very abundant in the Gulf of California, Polynesia, and India. The single living species is the _Awa_, or milkfish, _Chanos chanos_, largely used as food in Hawaii. Species of _Prochanos_ and _Chanos_ occur in the Cretaceous, Eocene, and Miocene. Allied to _Chanos_ is the Cretaceous genus _Ancylostylos_ (_gibbus_), probably the type of a distinct family, toothless and with many-rayed dorsal.

=The Hiodontidæ.=—The _Hiodontidæ_, or mooneyes, inhabit the rivers of the central portion of the United States and Canada. They are shad-like fishes with brilliantly silvery scales and very strong sharp teeth, those on the tongue especially long. They are very handsome fishes and take the hook with spirit, but the flesh is rather tasteless and full of small bones, much like that of the milkfish. The commonest species is _Hiodon tergisus_. No fossil _Hiodontidæ_ are known.

=The Pterothrissidæ.=—The _Pterothrissidæ_ are sea-fishes like _Albula_, but more slender and with a long dorsal fin. They live in deep or cold waters along the coasts of Japan, where they are known as gisu. The single species is _Pterothrissus gissu_. The fossil genus _Istieus_, from the Upper Cretaceous, probably belongs near the _Pterothrissidæ_. _Istieus grandis_ is the best-known species. Another ancient family, now represented by a single species, is that of the _Chirocentridæ_, of which the living type is _Chirocentrus dorab_, a long, slender, much compressed herring-like fish, with a saw-edge on the belly, found in the East Indies, in which region _Chirocentrus polyodon_ occurs as a fossil. Numerous fossil genera related to _Chirocentrus_ are enumerated by Woodward, most of them to be referred to the related family of _Ichthyodectidæ_ (_Saurodontidæ_). Of these, _Portheus_, _Ichthyodectes_, _Saurocephalus_ (_Saurodon_), and _Gillicus_ are represented by numerous species, some of them fishes of immense size and great voracity. _Portheus molossus_, found in the Cretaceous of Nebraska, is remarkable for its very strong teeth. Species of other genera are represented by numerous species in the Cretaceous of both the Rocky Mountain region and of Europe.

=The Ctenothrissidæ.=—A related family, _Ctenothrissidæ_, is represented solely by extinct Cretaceous species. In this group the body is robust with large scales, ctenoid in _Ctenothrissa_, cycloid in _Aulolepis_. The fins are large, the belly not serrated, and the teeth feeble. _Ctenothrissa vexillifera_ is from Mount Lebanon. Other species occur in the European chalk. In the small family of _Phractolæmidæ_ the interopercle, according to Boulenger, is enormously developed.

=The Notopteridæ.=—The _Notopteridæ_ is another small family in the rivers of Africa and the East Indies. The body ends in a long and tapering fin, and, as usual in fishes which swim by body undulations, the ventral fins are lost. The belly is doubly serrate. The air-bladder is highly complex in structure, being divided into several compartments and terminating in two horns anteriorly and posteriorly, the anterior horns being in direct communication with the auditory organ. A fossil _Notopterus_, _N. primævus_, is found in the same region.

=The Clupeidæ.=—The great herring family, or _Clupeidæ_, comprises fishes with oblong or herring-shaped body, cycloid scales, and feeble dentition. From related families it is separated by the absence of lateral line and the division of the maxillary into three pieces. In most of the genera the belly ends in a serrated edge, though in the true herring this is not very evident, and in some the belly has a blunt edge. Some of the species live in rivers, some ascend from the sea for the purpose of spawning. The majority are confined to the ocean. Among all the genera, the one most abundant in individuals is that of _Clupea_, the herring. Throughout the North Atlantic are immense schools of _Clupea harengus_. In the North Pacific on both shores another herring, _Clupea pallasi_, is equally abundant, and with the same market it would be equally valuable. As salted, dried, or smoked fish the herring is found throughout the civilized world, and its spawning and feeding-grounds have determined the location of cities.

The genus _Clupea_, of northern distribution, has the vertebræ in increased number (56), and there are weak teeth on the vomer. Several other genera are very closely related, but ranging farther south they have, with other characters, fewer (46 to 50) vertebræ. The alewife, or branch-herring (_Pomolobus pseudoharengus_), ascends the rivers to spawn and has become landlocked in the lakes of New York. The skipjack of the Gulf of Mexico, _Pomolobus chrysochloris_, becomes very fat in the sea. The species becomes landlocked in the Ohio River, where it thrives as to numbers, but remains lean and almost useless as food. The glut-herring, _Pomolobus æstivalis_, and the sprat, _Pomolobus sprattus_, of Europe are related forms.

Very near also to the herring is the shad (_Alosa sapidissima_) of the eastern coasts of America, and its inferior relatives, the shad of the Gulf of Mexico (_Alosa alabamæ_), the Ohio River shad (_Alosa ohiensis_), very lately discovered, the Allice shad (_Alosa alosa_) of Europe, and the Thwaite shad (_Alosa finta_). In the genus _Alosa_ the cheek region is very deep, giving the head a form different from that seen in the herring.

The American shad is the best food-fish in the family, peculiarly delicate in flavor when broiled, but, to a greater degree than occurs in any other good food-fish, its flesh is crowded with small bones. The shad has been successfully introduced into the waters of California, where it abounds from Puget Sound to Point Concepcion, ascending the rivers to spawn in May as in its native region, the Atlantic coast.

The genus _Sardinella_ includes species of rich flesh and feeble skeleton, excellent when broiled, when they may be eaten bones and all. This condition favors their preservation in oil as "sardines." All the species are alike excellent for this purpose. The sardine of Europe is the _Sardinella pilchardus_, known in England as the pilchard. The "Sardina de España" of Cuba is _Sardinella pseudohispanica_, the sardine of California, _Sardinella cærulea_. _Sardinella sagax_ abounds in Chile, and _Sardinella melanosticta_ is the valued sardine of Japan.

In the tropical Pacific occur other valued species, largely belonging to the genus _Kowala_. The genus _Harengula_ contains small species with very large, firm scales which do not fall when touched, as is generally the case with the sardines. Most common of these is _Harengula sardina_ of the West Indies. Similar species occur in southern Europe and in Japan.

In _Opisthonema_, the thread-herring, the last dorsal ray is much produced, as in the gizzard-shad and the tarpon. The two species known are abundant, but of little commercial importance. Of greater value are the menhaden, or the moss-bunker, _Brevoortia tyrannus_, inhabiting the sandy coasts from New England southward. It is a coarse and bony fish, rarely eaten when adult, although the young in oil makes acceptable sardines. It is used chiefly for oil, the annual yield exceeding in value that of whale-oil. The refuse is used as manure, a purpose for which the fishes are often taken without preparation, being carried directly to the cornfields. From its abundance this species of inferior flesh exceeds in commercial value almost all other American fishes excepting the cod, the herring, and the quinnat salmon.

One of the most complete of fish biographies is that of Dr. G. Brown Goode on the "Natural and Economic History of Menhaden."

Numerous other herring-like forms, usually with compressed bodies, dry and bony flesh, and serrated bellies, abound in the tropics and are largely salted and dried by the Chinese. Among these are _Ilisha elongata_ of the Chinese coast. Related forms occur in Mexico and Brazil.

The round herrings, small herrings which have no serrations on the belly, are referred by Dr. Gill to the family of _Dussumieriidæ_. These are mostly small tropical fishes used as food or bait. One of these, the Kobini-Iwashi of Japan (_Stolephorus japonicus_), with a very bright silver band on the side, has considerable commercial importance. Very small herrings of this type in the West Indies constitute the genus _Jenkinsia_, named for Dr. Oliver P. Jenkins, the first to study seriously the fishes of Hawaii. Other species constitute the widely distributed genera _Etrumeus_ and _Dussumieria_. _Etrumeus sardina_ is the round herring of the Virginia coast. _Etrumeus micropus_ is the Etrumei-Iwashi of Japan and Hawaii.

Fossil herring are plentiful and exist in considerable variety, even among the _Clupeidæ_ as at present restricted. _Histiothrissa_ of the Cretaceous seems to be allied to _Dussumieria_ and _Stolephorus_. Another genus, from the Cretaceous of Palestine, _Pseudoberyx_ (_syriacus_, etc.), having pectinated scales, should perhaps constitute a distinct subfamily, but the general structure is like that of the herring. More evidently herring-like is _Scombroclupea_ (_macrophthalma_). The genus _Diplomystus_, with enlarged scales along the back, is abundantly represented in the Eocene shales of Green River, Wyoming. Species of similar appearance, usually but wrongly referred to the same genus, occur on the coasts of Peru, Chile, and New South Wales. A specimen of _Diplomystus humilis_ from Green River is here figured. Numerous herring, referred to _Clupea_, but belonging rather to _Pomolobus_, or other non-Arctic genera, have been described from the Eocene and later rocks.

Several American fossil herring-like fishes, of the genus _Leptosomus_, as _Leptosomus percrassus_, are found in the Cretaceous of South Dakota.

Fossil species doubtfully referred to _Dorosoma_, but perhaps allied rather to the thread-herring (_Opisthonema_), being herrings with a prolonged dorsal ray, are recorded from the early Tertiary of Europe. Among these is _Opisthonema doljeanum_ from Austria.

=The Dorosomatidæ.=—The gizzard-shad, _Dorosomatidæ_, are closely related to the _Clupeidæ_, differing in the small contracted toothless mouth and reduced maxillary. The species are deep-bodied, shad-like fishes of the rivers and estuaries of eastern America and eastern Asia. They feed on mud, and the stomach is thickened and muscular like that of a fowl. As the stomach has the size and form of a hickory-nut, the common American species is often called hickory-shad. The gizzard-shad are all very poor food-fish, bony and little valued, the flesh full of small bones. The belly is always serrated. In three of the four genera of _Dorosomatidæ_ the last dorsal ray is much produced and whip-like. The long and slender gill-rakers serve as strainers for the mud in which these fishes find their vegetable and animal food. _Dorosoma cepedianum_, the common hickory-shad or gizzard-shad, is found in brackish river-mouths and ponds from Long Island to Texas, and throughout the Mississippi Valley in all the large rivers. Through the canals it has entered Lake Michigan. The Konoshiro, _Clupanodon thrissa_, is equally common in China and Japan.

=The Engraulididæ.=—The anchovies (_Engraulididæ_) are dwarf herrings with the snout projecting beyond the very wide mouth. They are small in size and weak in muscle, found in all warm seas, and making a large part of the food of the larger fish. The genus _Engraulis_ includes the anchovy of Europe, _Engraulis encrasicholus_, with similar species in California, Chile, Japan, and Australia. In this genus the vertebræ are numerous, the bones feeble, and the flesh tender and oily. The species of _Engraulis_ are preserved in oil, often with spices, or are made into fish-paste, which is valued as a relish. The genus _Anchovia_ replaces _Engraulis_ in the tropics. The vertebræ are fewer, the bones firm and stiff, and the flesh generally dry. Except as food for larger fish, these have little value, although existing in immense schools. Most of the species have a bright silvery band along the side. The most familiar of the very numerous species is the silver anchovy, _Anchovia browni_, which abounds in sandy bays from Cape Cod to Brazil. Several other genera occur farther southward, as well as in Asia, but _Engraulis_ only is found in Europe. Fossil anchovies called _Engraulis_ are recorded from the Tertiary of Europe.

=Gonorhynchidæ.=—To the _Isospondyli_ belongs the small primitive family of _Gonorhynchidæ_, elongate fishes with small mouth, feeble teeth, no air-bladder, small scales of peculiar structure covering the head, weak dentition, the dorsal fin small, and posterior without spines. The mesocoracoid is present as in ordinary _Isospondyli_. _Gonorhynchus abbreviatus_ occurs in Japan, and _Gonorhynchus gonorhynchus_ is found in Australia and about the Cape of Good Hope. Numerous fossil species occur. _Charitosomus lineolatus_ and other species are found in the Cretaceous of Mount Lebanon and elsewhere. Species without teeth from the Oligocene of Europe and America are referred to the genus _Notogoneus_. _Notogoneus osculus_ occurs in the Eocene fresh-water deposits at Green River, Wyoming. It bears a very strong resemblance in form to an ordinary sucker (_Catostomus_), for which reason it was once described by the name of _Protocatostomus_. The living _Gonorhynchidæ_ are all strictly marine.

In the small family of _Cromeriidæ_ the head and body are naked.

=The Osteoglossidæ.=—Still less closely related to the herring is the family of _Osteoglossidæ_, huge pike-like fishes of the tropical rivers, armed with hard bony scales formed of pieces like mosaic. The largest of all fresh-water fishes is _Arapaima gigas_ of the Amazon region, which reaches a length of fifteen feet and a weight of 400 pounds. It has naturally considerable commercial importance, as have species of _Osteoglossum_, coarse river-fishes which occur in Brazil, Egypt, and the East Indies. _Heterotis nilotica_ is a large fish of the Nile. In some or all of these the air-bladder is cellular or lung-like, like that of a Ganoid.

Allied to the _Osteoglossidæ_ is _Phareodus_ (_Dapedoglossus_), a group of large shad-like fossil fishes, with large scales of peculiar mosaic texture and with a bony casque on the head, found in fresh-water deposits of the Green River Eocene. In the perfect specimens of _Phareodus_ (or _Dapedoglossus_) _testis_ the first ray of the pectoral is much enlarged and serrated on its inner edge, a character which may separate these fishes as a family from the true _Osteoglossidæ_. It does not, however, appear in Cope's figures, none of his specimens having the pectorals perfect. In these fishes the teeth are very strong and sharp, the scales are very large and thin, looking like the scales of a parrot-fish, the long dorsal is opposite to the anal and similar to it, and the caudal is truncate. The end of the vertebral column is turned upward.

Other species are _Phareodus acutus_, known from the jaws; _P. encaustus_ is known from a mass of thick scales with reticulate or mosaic-like surface, much as in _Osteoglossum_, and _P. æquipennis_ from a small example, perhaps immature. _Phareodus testis_ is frequently found well preserved in the shales at Fossil Station, to the northwestward of Green River. Whether all these species possess the peculiar structure of the scales, and whether all belong to one genus, is uncertain.

In Eocene shales of England occurs _Brychætus muelleri_, a species closely related to _Phareodus_, but the scales smaller and without the characteristic reticulate or mosaic structure seen in _Phareodus encaustus_.

=The Pantodontidæ.=—The bony casque of _Osteoglossum_ is found again in the _Pantodontidæ_, consisting of one species, _Pantodon buchholzi_, a small fish of the brooks of West Africa. As in the _Osteoglossidæ_ and in the _Siluridæ_, the subopercle is wanting in _Pantodon_.

The _Alepocephalidæ_ are deep-sea herring-like fishes very soft in texture and black in color, taken in the oceanic abysses. Some species may be found in almost all seas below the depth of half a mile. _Alepocephalus rostratus_ of the Mediterranean has been long known, but most of the other genera, _Talismania_, _Mitchillina_, _Conocara_, etc., are of very recent discovery, having been brought to the surface by the deep-sea dredging of the _Challenger_, the _Albatross_, the _Blake_, the _Travailleur_, the _Talisman_, the _Investigator_, the _Hirondelle_, and the _Violante_.