Chapter VII: , is termed the endoskeleton. The latter consists (i.) of an (10)

In the remaining Teleostomi (ACTINOPTERYGII) the paired fins are invariably non-lobate, with abbreviate, multibasal endoskeletal supports. Fin-rays are the main support of both the median and paired fins. Jugular plates are usually replaced by branchiostegal rays, but both may co-exist. The Actinopterygii are the successors of the Crossopterygii in palaeontological sequence, and when the latter began to decline in Carboniferous and Permian times, the former, mainly represented by the earlier Chondrostei, had already become the dominant Fishes of the period.

ORDER II. CHONDROSTEI (ACIPENSEROIDEI).

In these Fishes, the oldest and the most primitive of the Actinopterygii, the fin-rays of the median fins still continue to retain their primitive numerical superiority over the radials, and the tail is heterocercal. There is a single dorsal and an anal fin, which, like the upper lobe of the caudal fin, are generally provided with fulcra. Pelvic fins abdominal. Squamation typically rhombic and ganoid. Vertebral column acentrous. So far as is known the chondrocranium is but little ossified, and the cranial bones are mainly dermal. The secondary pectoral girdle still includes a pair of infra-clavicles.

The Chondrostei are first represented in the Lower Devonian by the solitary Palaeoniscid genus _Cheirolepis_, a contemporary of the earliest Crossopterygii. They occur throughout the Mesozoic period, except in the Cretaceous, and also in the Eocene, and while steadily diminishing in number and variety they gradually approximate to their degenerate and in some respects highly specialised descendants, the Sturgeons and Paddle-Fishes of the existing Fish fauna. Of the seven families included in the group the Palaeoniscidae are the oldest and the most generalised. The Platysomidae are a specialised offshoot from the Palaeoniscidae, and, if they are rightly to be considered as Chondrostei, perhaps the same may be said of the problematic Belonorhynchidae. On {486}the other hand, there are certain features in the Catopteridae which indicate an approach to Fishes of an altogether more modern type. Finally, the Chondrosteidae represent a stage in a career of degeneration, the climax of which is reached by the modern Polyodontidae and Acipenseridae.

FAM. 1. PALAEONISCIDAE.[579]—Fishes with fusiform bodies, short dorsal and anal fins, and usually with a complete investment of articulating rhombic, rarely cycloid, ganoid scales (Fig. 283). Fulcra generally present at the bases of the median fins, and especially along the dorsal border of the upper caudal lobe. Ribs are not known to be present. Skull invested by a very complete series of paired dermal bones, which in number and disposition conform to the normal Teleostome type (Fig. 284). The secondary upper jaw includes both premaxillae and large maxillae; and, as a rule, both the dentary and splenial bones {487}of the lower jaw are dentigerous. Except for the absence of an interoperculum, the opercular series of bones is complete, including numerous branchiostegal rays. There is a single small median jugular plate.

The Palaeoniscidae are remarkable both for their individual and specific abundance and for their extensive range in time. Represented only by _Cheirolepis_ in the Middle Old Red Sandstone and Devonian, the family attained its maximum development in the later Palaeozoic rocks (Carboniferous and Lower Permian), became rare in the Mesozoic, finally dwindling away at the close of the Jurassic period. Their geographical distribution in the past is hardly less remarkable. In various geological formations they have been found in Great Britain and Ireland, in widely remote parts of continental Europe, and in North America, South Africa, and Australia. _Cheirolepis_, _Amblypterus_, _Canobius_, _Phanerosteon_, _Elonichthys_, _Cryphiolepis_, _Palaeoniscus_, and _Trissolepis_ are Palaeozoic genera. _Gyrolepis_, _Urolepis_, _Coccolepis_, _Oxygnathus_, and _Centrolepis_ are characteristic Mesozoic forms.

FAM. 2. PLATYSOMIDAE.[580]—More or less deep-bodied Fishes, with elongated dorsal and anal fins, a high head, short jaws, usually armed with bluntly conical tritoral teeth, and a complete investment of high, narrow, rhombic scales. They agree with the Palaeoniscidae in their osteology and in most other essential {488}features, and they flourished in large numbers during the Carboniferous and Permian periods. _Platysomus_ ranges from the Lower Carboniferous to the Upper Permian in Great Britain and continental Europe, and also occurs in the Carboniferous of North America. _Eurynotus_ (Fig. 285), and the singularly deep-bodied _Cheirodus_ (Fig. 286), in which pelvic fins are unknown, are British Carboniferous genera.

FAM. 3. BELONORHYNCHIDAE.—The systematic position of these Triassic forms is very doubtful, and it is by no means clear that they are Chondrostei at all.

FAM. 4. CATOPTERIDAE.—It is very probable that this widely-distributed Triassic family is an offshoot from the Palaeoniscidae. It agrees with the latter in the general character of the head and pectoral girdle and in the rhombic squamation, but differs from its progenitors and approaches the more modern Holostei in the semi-heterocercal condition of the tail, and in the approximate numerical agreement between the fin-rays and radialia of the dorsal and anal fins.[581]

{489}FAM. 5. CHONDROSTEIDAE.—This family affords an interesting annectant link between the Palaeoniscidae and their degenerate living representatives the Polyodontidae and Acipenseridae. They agree with the latter in the general shape of the body, the growth of a preoral rostrum, and in the relatively small size of their ventrally-placed and probably protrusible mouth (Fig. 287). The skin is entirely scaleless, except on the upper lobe of the caudal fin, where, as in _Polyodon_ and _Acipenser_, the primitive rhombic squamation and a series of fulcra are retained.

On the other hand, their relationship to the Palaeoniscidae is indicated by the general disposition of the dermal bones of the cranial roof, and the presence of a transverse row of supra-temporals and of an extensive series of branchiostegal rays (Fig. 288). The family is represented by _Chondrosteus_[582] from the Lower Lias of Dorset and Leicestershire, and _Gyrosteus_ from the Upper Lias of Yorkshire. From an evolutionary point of view it is significant that the Chondrosteidae do not make their appearance until the Palaeoniscidae are approaching extinction.

The two remaining families, the Polyodontidae and the Acipenseridae, agree in presenting a remarkable leaven of characters otherwise distinctive of the typical Elasmobranch, associated with certain primitive features which they have doubtless inherited from some remote ancestral stock common both to existing Elasmobranchs and to the other primary groups of Fishes, and also with others obviously due to degeneration.

{490}

The most interesting illustration of the first point is to be found in the condition of the primitive upper jaw which, especially in the Polyodontidae, is typically Elasmobranch in the median union of the palato-quadrate bars beneath the basis cranii, but Teleostome in the presence of a secondary upper jaw formed by two maxillae. Both families also agree in possessing an acentrous vertebral column which, if it does so far resemble that of Teleostomes in being potentially arco-centrous, nevertheless has a better developed series of distinct inter-dorsal and inter-ventral cartilages, regularly alternating with only partially bony basi-dorsals and basi-ventrals, than is to be met with in any other adult Fishes except Elasmobranchs. Primitive features are apparent in the presence of spiracles, sometimes associated with pseudobranchs; the presence in one family (Acipenseridae) of a hyoidean hemibranch supplied with blood directly from the ventral aorta, and the existence of a multi-valvular conus arteriosus and an intestinal spiral valve. Finally, the massive growth of the chondrocranium wholly devoid of cartilage bones, except in so far as they may be represented by splint-like membrane bones, the fragmentation of the investing dermal bones, the degeneration of the opercular skeleton and the loss of branchiostegal rays, and the almost complete disappearance of the primitive rhombic squamation, are probably to be regarded {491}as the outcome of a long-continued career of degeneration from some remote Palaeoniscid ancestor.

FAM. 6. POLYODONTIDAE.—The Polyodontidae are more generalised, and in some features decidedly more Selachioid than the Acipenseridae. Body fusiform and apparently scaleless, but the primitive squamation is still represented by isolated vestigial scales imbedded in the otherwise soft skin, and by a continuous series of rhombic scales on the upper caudal lobe, which also has a dorsal fringe of large fulcra.[583] Rostrum exceptionally long, spatulate or somewhat conical, with a rigid axis and thinner and more flexible margins. Barbels absent. Mouth wide, not spout-like. Pectoral fins devoid of spines. Two pairs of membrane-closed vacuities separate the paired dermal bones of the cranial roof (possibly parietals and frontals) from the more laterally-placed post-temporals and squamosals, and there are no median plates posterior to the orbits, nor any representatives of supra-temporals. A feeble suboperculum is retained in addition to a small rayed operculum. Hyoidean hemibranch completely suppressed. Two genera only are known, each with a single species.

The Paddle-Fish or Spoon-Bill, _Polyodon folium_ (Fig. 289) inhabits the rivers of the Southern States of North America, the Mississippi, Ohio, and Missouri, and their numerous tributary rivers and streams. A Fish of sluggish habits, _Polyodon_ feeds chiefly on mud and the minute organisms it contains, the exceptionally long gill-rakers probably forming an efficient filter to prevent the food particles escaping through the gill-clefts with the expiratory water current. The singular rostrum is apparently used for stirring up the mud when feeding, but in view of the muddy waters the Fish frequents, and the very small size of {492}the eyes, its value as a tactile organ must not be overlooked. _Polyodon_ may attain a length of 5 to 6 feet. The time of spawning varies, according to locality, from March to June. Nothing is known of the development of _Polyodon_. Young less than 6 to 8 inches in length are unknown, and specimens of this size are very rarely seen. The jaws are furnished with minute teeth until the Fish is about half-grown, when they become edentalous. Caviare is made from the eggs, and the centres at which this industry is carried on are chiefly situated along the course of the Mississippi. The second species, _Psephurus gladius_, inhabits the Yang-tse-Kiang and Hoangho rivers of China, and differs from _Polyodon_ in the conical shape of its rostrum and the smaller number and larger size of its fulcra. _Psephurus_ is stated to reach a length of 20 feet. The family is represented in the Eocene of Wyoming by the genus _Crossopholis_, which is note-worthy for the retention of trunk scales in the form of small, somewhat quadrate denticulated discs, arranged in oblique rows.

FAM. 7. ACIPENSERIDAE.—In the Sturgeon family the body is elongate, cylindrical, and somewhat bulky. Rostrum well developed and often massive, with a transverse row of simple or branched preoral barbels on its ventral surface. Mouth small and remarkably protrusible. Jaws devoid of teeth except in the larvae. As in the preceding family, the primitive rhombic squamation is confined to the upper lobe of the tail, which, like the dorsal and anal fins, is furnished with fulcra. Elsewhere the scales are represented by five longitudinal rows of large bony scutes and by intervening small scattered ossifications. The anterior dermal ray of the pectoral fin is stout and spine-like. The dermal bones of the cranial roof suturally articulate with one another to form a continuous shield, uninterrupted by lateral vacuities. A median dermal bone in the occipital region transmits the occipital sensory canal. The opercular series is represented only by an opercular bone.

The family includes but two genera, _Acipenser_ (Fig. 290) and _Scaphirhynchus_, and about twenty species, confined to the seas, estuaries, and rivers of the temperate and north temperate regions of the northern hemisphere. _Acipenser_ includes the more typical Sturgeons, and is distinguished by the presence of spiracles, and by the fact that the longitudinal rows of scutes remain distinct to the base of the caudal fin.

{493}

There are probably about fifteen species, but the exact number is uncertain. Sturgeons are abundant in the Black Sea, the Sea of Azov, the Caspian, and their tributary rivers, notably the Danube, Don, Dnieper, Ural, and Volga. They are also present in the rivers and on the coasts of Northern Europe and of China. Five species occur in North America, on the Atlantic and Pacific coasts, and in the rivers of these regions as well as in the Great Lakes.[584] One or two species are almost exclusively fresh-water, but most Sturgeons are migratory Fishes, living in the sea, but ascending rivers for spawning. Their food consists of worms, molluscs, the smaller Fishes and aquatic plants; and in feeding the mouth is protruded downwards in the form of a cylindrical, spout-like structure and thrust into the mud. The only species certainly known to frequent the British coasts is the common Sturgeon (_A. sturio_), which is also found in the Black Sea and the Mediterranean, and is abundant on the Atlantic coast of North America from Maine to South Carolina. The species occurs all round our coasts, more plentifully, perhaps, on the northern and eastern shores. In the spring and summer the Fish ascends the rivers, often to a considerable distance. Its presence has been recorded in the Severn, near Shrewsbury; in the Trent at Nottingham, and also, but not in recent years, in the Thames above London Bridge.[585] In this country the species is a "Royal Fish," and by an unrepealed Act of Edward II. it is enacted that "the King shall have the wreck of the sea throughout the realm, Whales and Great Sturgeons, except in certain places privileged by the King."[586] If not so large as some of its Russian relatives, _A. sturio_ often attains a great size. Even on {494}our own coasts the capture of individuals 8 to 10 feet in length has been recorded. The great Russian Sturgeon (_A. huso_), which is common in the Black Sea, the Sea of Azov and the Caspian, and in the rivers flowing into them, is the largest of all the Sturgeons, individuals weighing 2760 and 3200 pounds having been captured. The Sterlet (_A. ruthenus_), similarly distributed and often ascending the Danube to Vienna, is much smaller, rarely exceeding a length of three feet.

In Europe _A. sturio_ spawns about July, but in North America (Delaware river) during May. Small in size, the eggs are produced in enormous numbers, a single female, it is said, producing about 3,000,000 in one season. They are invested by a gelatinous sheath, so that they readily stick to one another or to other objects, and, when deposited, they adhere in streaks or sheet-like masses to the bed of the river. The young are hatched very early, about the third or fourth day in _A. sturio_, and in the Sterlet between the ninth and twelfth, the length of the larva then varying from 7 to 10 mm. When they are a few days old the larvae closely resemble those of existing Holostei except that the small opercular folds leave the gills freely exposed (Fig. 291). A shallow pigmented groove in front of the mouth apparently represents the sucker of the young _Amia_ and _Lepidosteus_. Although toothless in the adult, both the Sturgeon and Sterlet possess vestigial rudimentary, uncalcified, larval teeth, which in shape resemble the teeth of a Dog-Fish, consisting of a broad base and a sharp spine.

The Sturgeon is a Fish of considerable economic importance. The flesh is an article of food, and from the ovaries of certain Russian and American species thousands of hundredweights of {495}caviare are prepared annually. Large quantities of isinglass are obtained from the air-bladders, in the United States and in Russia. The organ is split open and washed; the inner lining is then stripped off and the bladder dried as rough isinglass.

The second genus, _Scaphirhynchus_, which includes the Shovel-nosed Sturgeons, differs from _Acipenser_ in the long, flattened, and almost spatulate shape of the rostrum, the suppression of the spiracles, and the union of the longitudinal rows of scutes beneath the dorsal fin to form a scaly armature completely investing the tail. The distribution of the genus affords an interesting parallel to that of the Polyodontidae. Of the four species, one (_S. platyrhynchus_) is common in the Mississippi valley and in the rivers of the Western and Southern States of North America, while the remaining species, also exclusively fresh-water, frequent the rivers of Tartary.

The Acipenseridæ are not known to occur earlier than the Tertiary. Scutes, pectoral spines and fragmentary bones, indistinguishable from the corresponding parts of existing species, have been recorded from the London Clay of the Isle of Sheppey (Lower Eocene), and from later Eocene deposits in the Isle of Wight and Hampshire; and also from the Pliocene of England (Red Crag of Suffolk) and Virginia.

ORDER III. HOLOSTEI (LEPIDOSTEOIDEI).

The Holostei include a large and somewhat heterogeneous assemblage of Fishes, most of which are now extinct. As a group they are by no means easy to define or delimit. Widely separated from the Chondrostei, there is little evidence of the existence of connecting links between the two groups, although in some respects the Catopteridae may be regarded as transitional. On the other side, however, the Holostei shade off almost imperceptibly into the Malacopterygian Teleostei. In different fossil and recent Holostei there may be traced the gradual acquisition of the more special Teleostean characters and the elimination of the more archaic features of their remote Teleostome ancestors; and in a general sense this may be taken as the key to the more salient attributes of the group. It is not suggested that all the families of Holostei are on the direct lines of Teleostean descent. Some families, like the Eugnathidae and {496}Amiidae, may possibly occupy this position, but others, such as the Pycnodonts, for example, seem to be highly specialised and terminal offshoots which have left no descendants. Of the more generalised features which different Holostei retain, mention may be made of the prevalence of rhombic scales which, like the dermal cranial bones, are generally invested by a variously ornamented coat of ganoin; the presence of fulcra, cheek-plates, post- or sub-orbital ossicles, and of a complex lower jaw, which includes dentigerous splenials; and the abdominal position of the pelvic fins. On the other hand, indication of advancing specialisation in the Teleostean direction are to be noted in the numerical agreement between the dermal fin-rays of the median fins and their supporting radialia, and in the character of the vertebral column. Some Holostei, especially the earlier forms, are acentrous, but between this primitive condition and the possession of well-ossified centra, associated with equally bony arcualia, almost every gradation is to be found. The chondrocranium is more or less completely replaced by cartilage bones corresponding to those generally present in Teleosts, while the palato-pterygoid cartilages, likewise modified by the growth of cartilage bones, separately articulate with the lateral ethmoid regions instead of meeting in a ventral symphysis beneath the basis cranii. With rare exceptions (_e.g._ certain Pycnodonts) the opercular skeleton is complete, and includes branchiostegal rays; and although a single gular plate is often present, it may be absent in entire families. Like so many other structures, the tail is in a transitional state: really heterocercal, but incipiently homocercal, it may be described as semi-heterocercal. Infra-clavicular plates no longer form part of the secondary pectoral girdle, their place being taken by cleithra which, as in most Teleosts, meet in a ventral symphysis.

Indications of transition are not wanting in the squamation in certain families, and may be seen in the partial or complete replacement of the rhombic type by thin, imbricated, cycloid scales. Lastly, the soft parts of the two surviving genera are not without features of similar significance. A multivalvular conus arteriosus, it is true, is still retained, but the spiral valve is vestigial, the spiracles are closed, and in the female of one genus (_Lepidosteus_) the gonoducts are peritoneal tubes, continuous, as in most Teleosts, with the investments of the ovaries.

{497}The Fishes here included in the Holostei constitute the Protospondyli and Aetheospondyli of Smith Woodward.[587] In the former group vertebral centra are either entirely absent, or, if present, their components in the form of alternating hypo- and pleuro-centra invariably remain distinct in the tail. The latter group has been instituted for the provisional reception of two highly specialised families of uncertain relationships, which differ from the Protospondyli in their higher grade of vertebral structure, the centra always being complete without any indication of distinct hypo- and pleuro-centra.

The Holostei first appear in the Permian, where they are represented by a single genus (_Acentrophorus_). During the Mesozoic period they were abundant in the Trias, reaching their maximum development and becoming the dominant Fishes of the period in the Jurassic. In the Cretaceous they began to decline, and in the Tertiaries became reduced to the two families which at the present day are the sole survivors of the group.

Of the six families of Protospondyli the Semionotidae are the oldest and most generalised, and the Macrosemiidae a closely allied group. The Pycnodontidae are a highly specialised and terminal offshoot. The Eugnathidae obviously lead to the Amiidae, and from the same stock it is probable that the Pachycormidae have been derived. The relations of the Aspidorhynchidae and Lepidosteidae (Aetheospondyli) are extremely doubtful. That the two families are allied seems probable, but beyond the possibility of a remote connection with the Protospondyli there is no clue to their ancestry.

FAM. 1. SEMIONOTIDAE.—Small-mouthed, fusiform or {498}deep-bodied Holosteans with rhombic scales, rarely, as in _Aetheolepis_, cycloid in the caudal region. All the fins possess fulcra. Teeth more or less conical, with a tendency to become tritoral in certain genera. Jugular plate present or absent. _Acentrophorus_ (Upper Permian); _Semionotus_ (Trias of England, Germany, S. Africa, and N. America); _Lepidotus_ (Fig. 292) (Trias of Germany, Jurassic of Europe and India, Cretaceous of Brazil); the deep-bodied _Dapedius_ (Lias of Dorset, Fig. 293), and _Aetheolepis_ (Jurassic of New South Wales) are characteristic genera.

FAM. 2. MACROSEMIIDAE.—Closely related to the Semionotidae, but with a more extended dorsal fin. _Macrosemius_ (Upper Jurassic of England, Germany, France); _Notagogus_ (Upper Jurassic of Naples, Bavaria, France); _Petalopteryx_ (Upper Cretaceous of Syria).

FAM. 3. PYCNODONTIDAE.—Highly specialised deep-bodied Fishes, with a small mouth and characteristic grinding or tritoral teeth. Scales rhombic. Fulcra absent. Dorsal and anal fins long. There is no jugular plate. The family ranges from the Lower Lias to the Lower Eocene, inclusive. _Mesodon_, _Mesturus_, _Gyrodus_, and _Microdon_ are Jurassic genera. _Coccodus_ and _Xenopholis_ occur in the Upper Cretaceous of Syria (Mount Lebanon), and _Pycnodus_ in various European Eocene formations.

FAM. 4. EUGNATHIDAE.—Large-mouthed, elongate fusiform {499}predaceous Fishes, with pointed teeth, rhombic scales, short dorsal and anal fins, a single jugular plate and prominent fulcra. The vertebral centra are represented by distinct hypo- and pleuro-centra, which may form complete alternating rings in the tail.

The family first appears in the Trias and ranges throughout the Jurassic period. _Eugnathus_ (Jurassic) and _Eurycormus_ (Upper Jurassic). _Caturus_ (Fig. 294) has a more extensive range, occurring in the Upper Trias of the Tyrol and in the Upper Jurassic of England and Bavaria. _Caturus_ and _Eurycormus_, with their relatively thin, imbricated, cycloid scales, which have lost the peg-and-socket articulation, form connecting links between the more typical _Eugnathus_ and the Amiidae.

FAM. 5. AMIIDAE.—Body fusiform and somewhat compressed. Scales uniformly thin, cycloid, and imbricated. Single dorsal fin long and low. Anal fin short. Tail nearly homocercal, with a rounded hinder margin. Fulcra absent from all the fins. Moderately large conical teeth are present on the premaxillae, maxillae, palatines and dentaries, and smaller teeth on the vomers, pterygoids, splenials and parasphenoid. Pre- and post-centra fused in the trunk, forming complete bony amphicoelous centra, but distinct in the tail. A single large jugular plate is present. In the solitary living species the air-bladder is cellular, and its afferent arteries are derived from a posterior aortic arch. Pyloric caeca absent. Two peculiar comb-like structures are present on the throat.

The Bow-Fin (_Amia calva_), the sole existing representative of the family, is abundant in the rivers and lakes of Central and Southern North America, including the great lakes Huron and {500}Erie. It is a voracious, carnivorous Fish, preying upon other Fish as well as upon fresh-water Crustaceans and Insects, very tenacious of life, and of no economic value. The male is smaller than the female, about 18 inches in length, and is distinguished by the presence of a round black spot, encircled by a margin of orange, at the base of the caudal fin (Fig. 295). The female may exceed 24 inches.

_Amia_ frequently rises to the surface, especially when the water is foul, and takes in large mouthfuls of air, and it is probable that the air is subsequently passed into the spacious cellular air-bladder which acts as a lung. The breeding season, during which the coloration of the Fish is more brilliant than at other times, lasts from the beginning of May to June, but it may begin and end somewhat earlier if the temperature be favourable. The Fish makes its way from the deeper water, where it has remained sluggish during the winter, to the spawning ground. This is usually at the swampy end of a lake where there is an abundance of aquatic herbage intersected by channels of clear water. There the Fish is said to circle round until the soft weeds and rootlets are bent and crushed aside, so as to leave an area having the appearance of a crude form of nest,[588] in which the eggs are deposited. They may be found in enormous numbers adhering to the leaves and rootlets of the weedy home. After oviposition the male remains on guard until the young are hatched out, when they appear to leave the nest in a body, still under the protection of their watchful parent. At all events a little later the male has been observed to be accompanied by a swarm of young fry, which he keeps together by circling round them. The development of the eggs is remarkably rapid. From the first cleavage of the egg to the hatching of the embryo the whole {501}process may be completed within from 4 to 8 days. When hatched the larvae are about 5 to 6 mm. long. They possess a large yolk sac and a preoral sucker for attachment. The pectoral fins are conspicuous structures before there is any trace of the pelvic fins.

The Amiidae, represented by _Megalurus_,[589] first appear in the Upper Jurassic of Dorset, France, and Bavaria. In the Cretaceous period the family is represented by _Amiopsis_. Species of _Amia_ occur in the Eocene of Europe and North America. In the former area the genus became extinct at the close of the Lower Miocene period, but in the latter _Amia calva_ still survives.

FAM. 6. PACHYCORMIDAE.—Large-mouthed, predaceous Amioid Fishes with a more or less prominent snout and a short dorsal fin. Scales rhombic but thin, rounded behind, and overlapping, sometimes absent. A single large jugular plate.

In the earlier forms (_e.g._ _Pachycormus_, Lias) the snout is but slightly produced, but in _Hypsocormus_ (Upper Jurassic), and {502}especially in _Protosphyraena_ (Cambridge Upper Greensand and the Cretaceous of Europe and North America), it becomes greatly elongated and associated with an exceptionally strong dentition.

FAM. 7. ASPIDORHYNCHIDAE.—Long-bodied Fishes, with a pointed preoral rostrum, sharp teeth, and deep rhombic scales. Fins small, the dorsal and anal being remote from the pelvic fins. Fulcra vestigial or absent. Jugular plates not known.

Two genera only are known. _Aspidorhynchus_ is a Jurassic form. _Belonostomus_ is Upper Jurassic and Cretaceous. Species of the latter genus have a very wide distribution (Europe, North and South America, and Australia).

FAM. 8. LEPIDOSTEIDAE.—Body elongate, with a relatively short caudal region. Tail semi-heterocercal. Scales rhombic, thick, ganoin-coated and articulated, not vertically elongated on the sides of the body. Dorsal and anal fins short and remote from the pelvic fins. Median fins with fulcra. Both the upper and lower jaws more or less elongated, forming a broad and depressed or a long tapering beak, near the anterior end of which the nostrils are placed. Eyes small. Vertebral centra well {503}ossified, opisthocoelous and fused with the neural arches. The metapterygoid bones have a secondary articulation with the skull.[590] Maxillae segmented into numerous pieces. Jugular plates absent. Branchiostegal rays reduced to three on each side. Teeth numerous, slender, and of unequal size. In the larger teeth the dentine is intricately folded. Pyloric caeca branched and compacted together into a gland-like mass. Air-bladder cellular, but its blood is not derived from a posterior aortic arch.

The only known genus is _Lepidosteus_, the existing species of which frequent the fresh waters of North America.[591] The common or Long-nosed Gar-Pike (_L. osseus_), remarkable for its long and slender beak, is generally abundant in the rivers and lakes of the United States from Vermont to the Rio Grande, and it may reach a length of five feet. The "Short-nosed Gar" (_L. platystomus_, Günther) has a much shorter and broader beak, and a similar distribution (Fig. 299). The "Great" or "Alligator Gar" (_L. viridis_, Günther) has a more southerly habitat, frequenting the rivers of the Southern States, Northern Mexico, and Cuba. It is by far the largest species, sometimes reaching a length of 8 to 10 feet.

_Lepidosteus_ is a voracious Fish, preying upon smaller Fishes, and, except in the breeding season, it frequents the deeper parts of the rivers or lakes. The Fish is constantly in the habit of rising to the surface and emitting bubbles of gas, either through the mouth or by the branchial clefts, and it is probable that this gas is air which has been previously swallowed at the surface and passed into the air-bladder. About May _Lepidosteus_ resorts in large numbers to shallower water, where the temperature is {504}higher, for the purpose of spawning, each female being attended by from one to four males.[592] During brief recurring periods of excitement, accompanied by convulsive lashing movements, the eggs and sperm are emitted. The eggs are extremely sticky, and adhere tenaciously to the rocks and stones on which they are deposited. In a few days the embryos hatch out, and at this stage the larva has a huge mouth surmounted by a terminal preoral disc, fringed with a row of marginal wart-like suckers (Fig. 300). The yolk sac is so large as greatly to hamper the movements of the larva; hence, by means of its suckers, the young _Lepidosteus_ attaches itself to surrounding objects, and remains almost entirely motionless for some little time after hatching. Later, about a fortnight after escaping from the egg, the yolk becomes completely absorbed, the suckers degenerate and eventually disappear, and the larva, freed from its load of nutritive reserve, assumes a more active life. After the absorption of the yolk the larvae of Mosquitos appears to form the exclusive diet of the young _Lepidosteus_ for some time, but very soon young Fishes are readily devoured.[593]

_Lepidosteus_ seems to have been abundant in Europe during the Eocene and Miocene periods, but became extinct before the Pliocene. In North America, also, the genus dates from the Eocene, and still survives.

ORDER IV. TELEOSTEI.

[For the account of this Order, see pp. 541 f.]

{505}CHAPTER XIX

DIPNEUSTI

SUB-CLASS III. DIPNEUSTI (DIPNOI).

These singularly interesting Fishes are distinguished by their more or less acutely lobate paired fins and their overlapping cycloid scales, and by the fact that the bony dermal fin-rays of the median fins are much more numerous than their supporting radialia. Tail heterocercal or apparently diphycercal. Nostrils inferior. Vertebral column acentrous. The radialia of the median fins articulate with the contiguous neural or haemal spines and agree with them numerically. Skull autostylic. Premaxillae and maxillae absent, but a secondary lower jaw is represented by certain dermal bones of which tooth-bearing splenials are the most important, the dentary bones being absent altogether, or, if present, toothless and small. The cranial dermal bones include median as well as paired lateral plates, but their relations to those of other Fishes are very obscure. Two opercular bones are always present, but branchiostegal rays are unknown. One of the most important diagnostic features is the dentition. All Dipneusti agree in possessing large tritoral dental plates supported by the palato-pterygoid and splenial bones. The secondary pectoral girdle includes only cleithra and infraclavicles (clavicles). There is a pelvic girdle. Claspers absent. Of the four families of Dipneusti, two, the Ctenodontidae and the Uronemidae, are exclusively Palaeozoic. The third, the Ceratodontidae, is Mesozoic, and still survives. The fourth, the Lepidosirenidae, is known only by two existing genera.

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FAM. 1. CTENODONTIDAE.—Body fusiform. Tail heterocercal or apparently diphycercal. Excluding the anal fin, which is always distinct, the remaining median fins are either distinct or continuous. Dental plates traversed by radiating transverse ridges terminating in rows of conical denticles (ctenodont dentition, Fig. 303). Vomerine teeth not known. Cranial bones numerous and small, and, like the squamation, with or without an investment of ganoin. Jugular plates present or absent.[594] The oldest genus is _Dipterus_[595] from the Old Red Sandstone of Scotland, where it is contemporaneous with the earliest Crossopterygii and also with the oldest known Actinopterygii (_Cheirolepis_). _Phaneropleuron_ (Old Red Sandstone of Scotland, Upper Devonian of Canada, Fig. 302), _Scaumenacia_ (Upper Devonian of Canada), _Ctenodus_ (Carboniferous of Great Britain and North {507}America), and _Sagenodus_ (Carboniferous of Great Britain and Lower Permian of Bohemia) belong to the same family.

FAM. 2. URONEMIDAE.—Body fusiform. Dentition non-ctenodont, consisting of patches of distinct rounded denticles with a row of basally-confluent conical denticles along the outer margin of each. Scales thin. All the median fins are continuous. Tail apparently diphycercal. Cranial dermal bones as in _Dipterus_. _Uronemus_[596] (Lower Carboniferous of Scotland), and perhaps _Conchopoma_[597] (Lower Permian of Prussia), are the only known genera.

The two remaining families possess certain features which cannot be affirmed to have existed in their extinct allies. Thus, both agree in exhibiting those striking and, so far as Fishes are concerned, unique modifications of the air-bladder and vascular system, and the olfactory organs,[598] which are more or less closely associated with air-breathing habits and indicate a marked convergence towards the Amphibia. Side by side with such indications of advancing specialisation in certain directions, ample evidence of a remote ancestry is to be seen in such primitive features as the presence of a spiral valve and a multi-valvular conus arteriosus, and in the short and simple alimentary canal. Of other points of agreement mention may be made of the absence of jugular plates, the presence of vomerine teeth, the continuity of all the median fins, and the apparently diphycercal but probably gephyrocercal character of the tail.

FAM. 3. CERATODONTIDAE.—Body elongated and compressed. Scales large, thin, non-ganoid, and partially enclosed in dermal pouches. Paired fins biserial. Chondrocranium complete. Dermal bones wholly devoid of ganoin, reduced in number but increased in size. Circumorbital bones present. Dental plates oval, crescentic or triangular, traversed by several radiating enamelled ridges, terminating in smooth or feebly denticulated biting margins. Lower jaw with a small toothless dentary on each side. The hyoid arch includes a small hyomandibular and a hypo-hyal in addition to a cerato-hyal. Branchial arches five in number and bisegmented. The gills exhibit little evidence of {508}degeneration. Hyo-branchial cleft open, and associated with a pseudobranch. The first four branchial arches carry holobranchs. Air-bladder single. Young not provided with cutaneous gills. Two genera only are known, the Mesozoic _Ceratodus_ and the still living _Neoceratodus_. The former genus includes numerous species, for the most part known only by their dental plates, and has a remarkably wide distribution in different geological formations. Species occur in the Trias of England, Germany, India, South Africa (Upper Karoo strata), and also, but more rarely, in certain Jurassic deposits in England and in Colorado.[599] _Neoceratodus_ is represented by a solitary species, _N. forsteri_[600] (Fig. 304, A), which is now restricted to the Burnett and Mary rivers in Queensland. A somewhat wider distribution of the species in recent times is indicated by the presence of teeth in the later Tertiary (alluvial) deposits of Darling Downs, near the borders of New South Wales.

The _Neoceratodus_[601] of the Burnett frequents the comparatively stagnant pools or water-holes which alternate with shallow runs and are usually full of water all the year round. In these pools, filled with a rich growth of aquatic vegetation, and often the favourite haunt of the Platypus (_Ornithorhynchus_), the Fish is fairly abundant. Inactive and sluggish in its habits, usually lying motionless on the bottom, the Fish is easily captured by the natives with hand-nets or baited hooks. _Neoceratodus_ lives on fresh-water Crustaceans, worms, and molluscs, and to obtain them it crops the luxuriant vegetation of the water-holes much in the same way that a Polychaet or a Holothurian swallows sand for the sake of the included nutrient particles. Apparently the air-bladder is a functional lung at all times, acting in conjunction with the gills. At irregular intervals the Fish rises to the surface and protrudes its snout in order to empty its lung and take in fresh air. While doing so the animal makes a peculiar grunting noise, "spouting" as the local fishermen call it, which may be heard at night for some distance, and is probably caused by the forcible expulsion of air through the mouth. Useful as the lung is as a breathing organ under normal conditions, there can be little doubt that its value as such is much greater whenever gill-breathing becomes difficult or impossible.

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This seems to be the case during the hot season, when the water becomes foul from the presence of decomposing animal or vegetable matter. Semon records a striking illustration of this in the case of a partially dried-up water-hole, in which the water had become so foul that it was full of dead fishes of various kinds. Fatal as these conditions were to ordinary Fishes, _Neoceratodus_ not only survived but seemed to be quite healthy and fresh. Such observations are of exceptional interest. Not only do they afford a clue to the conditions of life which, in the course of time, probably led to lung-breathing in _Neoceratodus_, but they also suggest the possibility that a similar environment has been conducive to the evolution of air-breathing Vertebrates from gill-breathing and Fish-like progenitors. In spite of its pulmonary respiration, _Neoceratodus_ more closely resembles the typical Fishes in its habits than any other Dipneusti. It lives all the year round in the water. There is no evidence that it ever becomes dried up in the mud, or passes into a summer sleep in a cocoon, and the well-developed condition of its gills suggest that these organs play a more important rôle in breathing than in either _Protopterus_ or _Lepidosiren_. The Fish is not known to leave the water, and the paired fins, useful no doubt as paddles, are quite incapable of supporting the bulky body on terra firma. In fact, when _Neoceratodus_ is taken out of its natural element it seems to be more helpless than most other Fishes, and, in spite of its capacity for lung-breathing, soon dies unless kept moist by artificial means. Spawning takes place from April to November, principally in September and October. The eggs, invested by a jelly-like coat, secreted by the oviducal walls, are deposited {511}not in a nest, but singly amongst aquatic vegetation, and, as they are not adherent, it is probable that they finally rest on the mud. The early developmental stages exhibit a general resemblance to those of Amphibia. There is no larval metamorphosis, and at no period does the young _Neoceratodus_ (Fig. 305) possess cutaneous gills or a cement organ. The tail is apparently diphycercal from the first, and the pelvic limbs do not appear until about six weeks after the pectoral members. It is interesting to note that the dental plates are first represented by lines or patches of separate denticles (non-ctenodont), which subsequently fuse basally (ctenodont) before the adult condition is reached.[602] _Neoceratodus_ is stated to grow to a length of 5 to 6 feet.

FAM. 4. LEPIDOSIRENIDAE.—Body elongate, cylindrical and more or less Eel-like, with small cycloid scales completely enclosed in the skin. Paired fins so acutely lobate as to present the appearance of tapering cylindrical filaments, equally devoid of scales and fin-rays. In a general way the cranial dermal bones correspond with those of _Neoceratodus_, but the place of the posterior median bone is taken by a large, gable-like fronto-parietal bone, situated internal to the head muscles, and in direct relation with the chondrocranium, which is largely aborted in the interorbital region. Circumorbital bones absent. Opercular bones much reduced. Lower jaw without dentary plates. Palatine and splenial dental plates with three non-denticulate, trenchant ridges. Hyoid arch consists of cerato-hyals only. Hyoidean cleft closed. Certain of the anterior branchial arches devoid of branchial filaments; when present the latter are leaf-like and free. Air-bladder a double lung. There is a larval metamorphosis, and the young possess cutaneous gills. The family includes two genera, _Protopterus_ and _Lepidosiren_. In the former genus the paired fins are either uniserial or they consist of axial mesomeres only; there are six branchial arches and five clefts; and the larval gills are usually retained as vestiges throughout life. In _Lepidosiren_ the paired fins are reduced to the segmented axis, without pre- or post-axial radials. There are five branchial arches and four clefts, and the cutaneous gills disappear soon after the larval metamorphosis.

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_Protopterus_ has a wide distribution over the middle portion of the great African continent, ranging from the river Senegal and the White Nile on the north to the Congo basin, Lake Tanganyika, and the Zambesi on the south. Three species are known, _P. annectens_ (Fig. 304), _P. aethiopicus_, and _P. dolloi_. _Protopterus_[603] is usually found in marshes in the vicinity of rivers. Voracious in its habits the Fish is mainly carnivorous, subsisting principally on Frogs, worms, insects, and crustaceans. It is by no means averse to preying upon its own kind, and if several of these Fishes are confined in the same aquarium they are apt to give free vent to their cannibal instincts by biting off the tails or limbs of their fellows. The missing parts are soon regenerated, but the new members are usually somewhat abnormal, the tail, for instance, never regaining its original length, while a new pectoral limb may be bifid or even trifid.[604] The tail is the principal organ of locomotion, and by its means the Fish is capable of remarkably quick, agile movements. When slowly moving over the bottom of an aquarium the paired limbs are observed to move to and fro on opposite sides alternately in a somewhat bipedal fashion. The limbs are useless for swimming, although it is possible that they may be helpful in creeping over the bottom, or in balancing, or as tactile organs. _Protopterus_ is said to breathe by its lungs as well as by its gills, and to rise to the surface at short intervals to take in fresh air.

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In the dry seasons the marshes in which _Protopterus_ lives become dried up, and to meet this adverse change in its surroundings the Fish hibernates, or passes into a summer sleep, until the next rainy season brings about conditions more favourable to active life. Preparatory to this summer sleep, and before the ground becomes too hard, the Fish makes its way into the mud to a depth of about 18 inches, and there coils itself up in a flask-like enlargement (Fig. 307) at the bottom of the burrow, which is lined by a capsule of hardened mucus secreted by the glands of the skin.[605] The mouth of the flask is closed by the capsular wall or lid, which is perforated by a small aperture. The margins of this aperture are pushed inwards, so as to form a tubular funnel for insertion between the lips of the Fish. While encapsuled in its cocoon the Fish is surrounded by a soft slimy mucus, no doubt for the purpose of keeping the skin moist, and its lungs are the sole breathing organs, the air passing from the open mouth of the burrow through the hole in the lid directly to the mouth of the animal. The nutrition of the dormant Fish is effected by the absorption of the fat stored up about the kidneys and gonads, somewhat after a fashion not unknown in the fat-bodies of Insects and the hibernating glands of Rodents. Even portions of the caudal muscles undergo fatty degeneration, and thus, in a way which recalls the mode of nutrition of the Salmon during the breeding season, and of the Tadpole during its metamorphosis, a further store of nutritive material becomes available for the sustenance of the Fish during {514}its long summer nap. It is highly probable that the exceptionally numerous leucocytes act as carriers in the work of transporting the fatty particles to the different organs and tissues of the body. The length of the summer sleep naturally varies with the duration of the dry season, and probably it lasts on an average nearly half the year (August to December). The cocoons, imbedded in an outward casing of hardened mud, have often been brought to Europe, and when placed in water of suitable temperature the long torpid _Protopterus_ escapes from its prison in a perfectly healthy condition, and resumes its partly branchial and partly pulmonary mode of breathing. The negroes of the West Coast of Africa are very partial to these Fishes, which they dig out of the dried marshes and preserve in their clumps of mud for food. With the advent of the rainy season, when the marshes become flooded, the _Protopterus_ emerges from its cocoon, and returning to its former active life, soon enters upon the task of reproducing its kind. The important observations of Budgett[606] have thrown much light on the curious breeding habits and development of these Fishes. The Fish makes a nest near the edge of a swamp. The nest is simply a hole of irregular shape, about a foot in depth, filled with water and surrounded by long grass (Fig. 308). There is no lining to the nest, and the eggs are deposited on the bare mud. Until the eggs are hatched, which occurs about the eighth day, and while the larvae are in the nest, the male remains on guard, and is apt to bite severely an incautious intruder. Probably with the view of aerating the eggs the water is continually lashed about by the tail of the guardian parent. The male has no trace of the peculiar vascular filaments which adorn the pelvic limbs of the male _Lepidosiren_ during the breeding season. The early developmental stages are similar in their main outlines to those of _Neoceratodus_, but the young are very different. When the young _Protopterus_ (Fig. 309) is hatched it is provided with a crescentic glandular sucker or cement-organ, situated on the under side of the head behind the mouth, by means of which the larva attaches itself to the sides of the nest, or of the vessel in which it is confined, much in the same way as the young _Lepidosteus_, and probably for the same reason. It may be remarked that the sucker agrees in structure, position, and function with that found in Amphibian tadpoles, but it differs both in position and structure from its preoral analogue in the young of _Acipenser_, _Amia_, and _Lepidosteus_.

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A month-old larva has much the aspect of a larval Newt. It has four pairs of vascular plumose cutaneous gills (Fig. 309), which are retained as vestiges for a long time or even throughout life, and two pairs of synchronously-developed limbs. As an interesting instance of a nocturnal and protective change of colour, it may be mentioned that the dark chromatophores of the skin of the larva expand in the day-time and the young Fish becomes darker in colour, and therefore less conspicuous when seen against a background of black mud or soil. At night the contraction of the colour-sacs renders the larva more transparent and probably less easily {516}visible than if opaque. The commencement of pulmonary respiration is coincident with the degeneration of the cutaneous gills, which takes place about seven weeks after the deposition of the eggs, and about a month after the larvae leave the nest. _Protopterus_ is said to attain a length of six feet.

_Lepidosiren paradoxa_,[607] probably the only species of the genus, is confined to South America. It occurs along the course of the main Amazon river, entering some of its larger affluents, such as the Ucayale, the Madeira, the Rio Negro, and the Tapajóz, and also in the Chaco Boreal to the west of the Upper Paraguay river. The home of the _Lepidosiren_ (or "Lolach," as the natives call the Fish) of the Chaco country is to be found in the wide-spreading marshes and swamps, which for a great part of the year are almost choked by a luxuriant growth of their own peculiar vegetation and covered by a floating carpet of surface weeds, with here and there deeper and clearer water and slow-flowing streams. In the dry season the water gradually shrinks and the swamps eventually become dried up. Of sluggish habits, the Fish wriggles slowly about at the bottom of the swamp like an Eel, using its hind limbs in an irregular bipedal fashion as it wends its way through the dense network of subaqueous plants. _Lepidosiren_ is not exclusively carnivorous. The large fresh-water snail, _Ampullaria_, which lives in the swamps in enormous numbers, seems to be its favourite food; but masses of confervoid Algae are also eaten, and in its earlier stages it is probable that the Fish is more herbivorous than carnivorous. The Jacare (_Caiman sclerops_) feeds on _Lepidosiren_, and this fact, and probably also the cannibal habits of the Fish itself, may explain the capture of specimens with mutilated tails and regenerated, branched, pectoral limbs. Like other living Dipneusti, _Lepidosiren_ rises to the surface to breathe. The intervals are, however, very variable, and no doubt depend on the relative purity or impurity of the water. Both expiration and inspiration are said to take place through the mouth. The snout is protruded on the surface, and the creature expires. After being withdrawn for a moment the head is again projected, and inspiration takes place through the partially open lips.

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When the Fish finally sinks a few bubbles of surplus air escape through the gill-clefts. A nocturnal and protective change of colour, similar to that described in _Protopterus_, has been observed, and although most strikingly manifest in the larvae, it also occurs in individuals of older growth. The flesh is much esteemed as food by the Indians, who wade into the swamps and transfix the Fishes with spears. During the rainy season the _Lepidosiren_ eats voraciously, and a reserve of fat is stored up in the tissues. Like its African relative, the Fish ceases to feed on the approach of the dry season, and eventually hibernates at the dilated extremity of a deep tubular burrow, the entrance to which is plugged by a small lump of clay perforated by several round holes. On the rising of the water at the next rainy season the _Lepidosiren_ pushes out the plug and soon emerges from its burrow.[608] The breeding season begins soon after the escape of the Fish. The eggs are deposited in nests in the form of underground burrows excavated in the black peaty soil at the bottom of the swamp, with an entrance about 4-5 inches wide. At a depth of about a foot the burrow takes a horizontal course, its total length varying from 2-5 feet. After the eggs are laid the male remains to guard them. During the breeding season the pelvic limbs of the male enlarge and become covered by a rich growth of highly vascular, blood-red filaments 2-3 inches in length[609] (Fig. 310). The use of these curious structures is uncertain, but it is not improbable that they act as accessory gills to enable the male to guard the eggs in the nest without being forced to resort to the surface to breathe air. The development is essentially similar to that of _Protopterus_. The larva (Fig. 311) has four pairs of cutaneous gills in relation with the first, second, third, and fourth branchial arches, inclusive, the first three pairs being the homologues of the cutaneous gills of the tailed Amphibia; and also a cement-organ {518}which disappears shortly before the larval metamorphosis. At that period the circulation in the cutaneous gills becomes sluggish, and very soon these organs completely atrophy. About the same time the hyo-branchial cleft closes up, as in _Protopterus_. The young _Lepidosiren_ soon begins to breathe air and to become more active and lively in its habits.[610] The adult may attain a length of four feet.