Chapter XXI: The Fishes of the Deep Sea (2)

Eight species are known from the southern parts of the European seas, and one from the Moluccas; they do not appear to exceed a length of five feet. According to the observations of E. P. Wright, some of the species at least live at a considerable depth, perhaps at a greater depth than any of the other known Sharks. The Portuguese fishermen fish for them in 400 or 500 fathoms with a line of some 600 fathoms in length. The Sharks caught were specimens of _Centrophorus coelolepis_, from three to four feet long. “These sharks, as they were hauled into the boat, fell down into it like so many dead pigs; there was not the smallest motion of their bodies. There can be no reasonable doubt that they were inhabitants of the same great depth as _Hyalonema_” and that, in fact, they were killed by being dragged to the surface from the pressure of water under which they lived. The dermal productions of some of the species have a very peculiar form, being leaf-shaped, pedunculate, or ribbed, or provided with an impression.

SPINAX.--Each dorsal fin with a spine. Teeth of the lower jaw
with the point so much turned aside that the inner margin of
the tooth forms the cutting edge. Upper teeth erect, each with
a long-pointed cusp and one or two small ones on each side.
Spiracles wide, superior, behind the eye.

Three small species from the Atlantic and the southern extremity of America. _Centroscyllium_ is an allied genus from the coast of Greenland.

SCYMNUS.--Two short dorsal fins without spine, the first at a
considerable distance from the ventrals. Dermal productions
uniformly small. Nostrils at the extremity of the snout. Upper
teeth small, pointed; lower much larger, dilated, erect,
triangular, not very numerous. Spiracles wide.

A single species, _S. lichia_, is rather common in the Mediterranean and the neighbouring parts of the Atlantic.

LÆMARGUS.--All the fins small; two dorsal fins, without spine,
the first at a considerable distance from the ventrals. Skin
uniformly covered with minute tubercles. Nostrils near the
extremity of the snout. The upper teeth small, narrow, conical;
the lower teeth numerous, in several series, the point so much
turned aside that the inner margin forms a cutting, non-serrated
edge. Jaws feeble. Spiracles of moderate width.

The “Greenland Shark” is an inhabitant of the Arctic regions, but rarely straying to the latitudes of great Britain; it grows to a length of about 15 feet, and, although it never or but rarely attacks man, is one of the greatest enemies of the whale, which is often found with large pieces bitten out of the tail by this Shark. Its voracity is so great that, according to Scoresby, it is absolutely fearless of the presence of man whilst engaged in feeding on the carcass of a whale, so that it can be pierced through with a spear or knife without being driven away. It is stated to be viviparous, and to produce about four young at a birth.

ECHINORHINUS.--Two very small dorsal fins, without spine, the
first opposite to the ventrals. Skin with scattered large round
tubercles. Nostrils midway between the mouth and the end of the
snout. Teeth equal in both jaws, very oblique, the point being
turned outwards; several strong denticulations on each side of
the principal point. Spiracles small.

The “Spinous Shark” is readily recognised by the short bulky form of its body, short tail, and large spinous tubercles. It is evidently a ground-shark, which probably lives at some depth and but accidentally comes to the surface. More frequently met with in the Mediterranean, it has been found several times on the south coast of England, and near the Cape of Good Hope.

_Euprotomicrus_ and _Isistius_ are two other genera of this family; they are pelagic and but little known.

NINTH FAMILY--RHINIDÆ.

_No anal fin; two dorsal fins. Spiracles present. Pectoral fins large, with the basal portion prolonged forwards, but not grown to the head._

RHINA.--Head and body depressed, flat; mouth anterior.
Gill-openings rather wide, lateral, partly covered by the base
of the pectoral. Spiracles wide, behind the eyes. Teeth conical,
pointed, distant. Dorsal fins on the tail.

The “Angel-fish,” or “Monk-fish” (_Rh. squatina_), approaches the Rays as regards general form and habits. Within the temperate and tropical zones it is almost cosmopolitan, being well known on the coasts of Europe, eastern North America, California, Japan, South Australia, etc.; it does not seem to exceed a length of five feet; it is viviparous, producing about twenty young at a birth.

Extinct forms, closely allied to the “Angel-fish,” are found in the Oolite, and have been described as _Thaumas_. The carboniferous genus, _Orthacanthus_, may have been allied to this family, but it was armed with a spine immediately behind the head.

TENTH FAMILY--PRISTIOPHORIDÆ.

_The rostral cartilage is produced, into an exceedingly long, flat lamina, armed along each edge with a series of teeth_ (saw).

These Sharks resemble so much the common Saw-fishes as to be easily confounded with them, but their gill-openings are lateral, and not inferior. They are also much smaller in size, and a pair of long tentacles are inserted at the lower side of the saw. The four species known (_Pristiophorus_) occur in the Australian and Japanese seas.

_Squaloraja_, from the Lias, is supposed to have its nearest affinities to this family.

B. BATOIDEI--RAYS.

In the typical Rays the body is excessively depressed, and forms, with the expanded pectoral fins, a circular or sub-rhomboidal disk, of which the slender tail appears as a more or less long appendage. In the two families which we shall place first (_Pristidæ_ and _Rhinobatidæ_), the general habit of the body still resembles that of the Sharks, but the gill-openings are ventral, as in the true Rays; the anal fin is invariably absent, and the dorsal fins, if developed, are placed on the tail. The mode of life of those fishes is quite in accordance with the form of their body. Whilst the species with a shark-like body and muscular tail swim freely through the water, and are capable of executing rapid and sustained motions, the true Rays lead a sedentary life, moving slowly on the bottom, rarely ascending to the surface. Their tail has almost entirely lost the function of an organ of locomotion, acting in some merely as a rudder. They progress solely by means of the pectoral fins, the broad and thin margins of which are set in an undulating motion, entirely identical with that of the dorsal and anal fins of the _Pleuronectidæ_. They are exclusively carnivorous, like the Sharks, but unable to pursue and catch rapidly-moving animals; therefore they feed chiefly on molluscous and crustaceous animals. However, the colour of their integuments assimilates so closely that of their surroundings, that other fishes approach them near enough to be captured by them. The mouth of Rays being entirely at the lower surface of the head, the prey is not directly seized with the jaws; but the fish darts over its victim so as to cover and hold it down with its body, when it is conveyed by some rapid motions to the mouth.

Rays do not descend to the same depth as Sharks; with one exception,[37] at least, none have been known to have been caught by a dredge worked in more than 100 fathoms. The majority are coast fishes, and have a comparatively limited geographical range, none extending from the northern temperate zone into the southern. However, some, if not all the species of the family _Myliobatidæ_, which includes the giants of this division of Plagiostomes, have a claim of being included among the Pelagic fishes, as they are frequently met with in the open ocean at a great distance from the shore. It is probable that the occurrence of such individuals in the open sea indicates the neighbourhood of some bank or other comparatively shallow locality. Many species are exclusively confined to fresh water, and occur far inland, especially in tropical America.

The majority are oviparous. All have five pairs of gill-openings. The number of known species is about the same as that of Sharks, viz. 140.

FIRST FAMILY--PRISTIDÆ.

_The snout is produced into an exceedingly long flat lamina, armed with a series of strong teeth along each edge_ (saw).

PRISTIS.--Body depressed and elongate, gradually passing into
the strong and muscular tail. Pectoral fins, with the front
margins quite free, not extending to the head. No tentacles
below the saw. Teeth in the jaws minute, obtuse. Dorsal fins
without spine, the first opposite or close to the base of the
ventrals.

“Saw-fishes.” Abundant in tropical, less so in sub-tropical seas. They attain to a considerable size, specimens with a saw 6 feet long and 1 foot broad at the base not being of uncommon occurrence. The saw, which is their weapon of attack, renders them most dangerous to almost all the other large inhabitants of the ocean. Its endoskeleton consists of three, sometimes five, rarely four, hollow cylindrical tubes, placed side by side, tapering towards the end, and incrusted with an osseous deposit. These tubes are the rostral processes of the cranial cartilage, and exist in all Rays, though in them they are shorter and much less developed. The teeth of the saw are implanted in deep sockets of the hardened integument. The teeth proper, with which the jaws are armed, are much too small for inflicting wounds or seizing other animals. Saw-fishes use this weapon in tearing pieces of flesh off an animal’s body or ripping open its abdomen. The detached fragments or protruding soft parts are then seized by them and swallowed. Five distinct species of Saw-fishes are known.

Saws of extinct species have been found in the London clay of Sheppey and in the Bagshot sands.

SECOND FAMILY--RHINOBATIDÆ.

_Tail strong and long, with two well-developed dorsal fins, and a longitudinal fold on each side; caudal developed. Disk not excessively dilated, the rayed portion of the pectoral fins not being continued to the snout._

RHYNCHOBATUS.--Dorsal fins without spine, the first opposite to
the ventrals. Caudal fin with the lower lobe well developed.
Teeth obtuse, granular, the dental surfaces of the jaws being
undulated.

Two species, _Rh. ancylostomus_ and _Rh. djeddensis_, are very common on the tropical coasts of the Indian Ocean. They feed on hard-shelled animals, and attain scarcely a length of 8 feet.

RHINOBATUS.--Cranial cartilage produced into a long rostral
process, the space between the process and pectoral fin
being filled by a membrane. Teeth obtuse, with an indistinct
transverse ridge. Dorsal fins without spine, both at a great
distance behind the ventral fins. Caudal fin without lower lobe.

Numerous on the coasts of tropical and sub-tropical seas; about twelve species. _Trygonorhina_ is an allied genus from South Australia.

The oolitic genus _Spathobatis_ is scarcely distinct from _Rhinobatus_; and another fossil from Mount Lebanon has been actually referred to this latter genus. _Trigorhina_ from Monte Postale must be placed here.

THIRD FAMILY--TORPEDINIDÆ.

_The trunk is a broad, smooth disk. Tail with a longitudinal fold on each side; a rayed dorsal fin is generally, and a caudal always, present. Anterior nasal valves confluent into a quadrangular lobe. An electric organ composed of vertical hexagonal prisms between the pectoral fins and the head._

“Electric Rays.” The electric organs with which these fishes are armed are large, flat, uniform bodies, lying one on each side of the head, bounded behind by the scapular arch, and laterally by the anterior crescentic tips of the pectoral fins. They consist of an assemblage of vertical hexagonal prisms, whose ends are in contact with the integuments above and below; and each prism is subdivided by delicate transverse septa, forming cells, filled with a clear, trembling, jelly-like fluid, and lined within by an epithelium of nucleated corpuscles. Between this epithelium and the transverse septa and walls of the prism there is a layer of tissue on which the terminations of the nerves and vessels ramify. Hunter counted 470 prisms in each battery of _Torpedo marmorata_, and demonstrated the enormous supply of nervous matter which they receive. Each organ receives one branch of the Trigeminal nerve and four branches of the Vagus, the former, and the three anterior branches of the latter, being each as thick as the spinal chord (electric lobes). The fish gives the electric shock voluntarily, when it is excited to do so in self-defence or intends to stun or to kill its prey; but to receive the shock the object must complete the galvanic circuit by communicating with the fish at two distinct points, either directly or through the medium of some conducting body. If an insulated frog’s leg touches the fish by the end of the nerve only, no muscular contractions ensue on the discharge of the battery, but a second point of contact immediately produces them. It is said that a painful sensation may be produced by a discharge conveyed through the medium of a stream of water. The electric currents created in these fishes exercise all the other known powers of electricity: they render the needle magnetic, decompose chemical compounds, and emit the spark. The dorsal surface of the electric organ is positive, the ventral surface negative.

[The literature on the electric organ of Torpedo is very
extensive. Here may be mentioned _Lorenzini_, “Osservazioni
intorno alle Torpedini,” (1678); _Walsh_, “On the Electric
Property of the Torpedo,” in Philos. Trans., 1773; _Hunter_,
“Anatomical Observations on the Torpedo,” _ibid._;
_Davy_, “Observations on the Torpedo,” in Philos. Trans.,
1834; _Matteucci_ and _Savi_, “Traité des Phénomènes
Electro-Physiologiques,” 1844.]

Of the genus _Torpedo_ six species are known, distributed over the Atlantic and Indian Oceans; three of them are rather common in the Mediterranean, and one (_T. hebetans_) reaches the south coast of England. They attain to a width of from two to three feet, and specimens of that size are able to disable by a single discharge a full-grown man, and, therefore, may prove dangerous to bathing persons. Other genera, differing from _Torpedo_ in the position and structure of some of the fins, are found in other tropical and sub-tropical seas, viz. _Narcine_, _Hypnos_, _Discopyge_ (Peru), _Astrape_, and _Temera_. All, like electric fishes generally, have a naked body.

A large fish, of the general appearance of a Torpedo, has been found at Monte Bolca; and _Cyclobatis_, from the upper cretaceous limestone of Lebanon, is probably another extinct representative of this family.

FOURTH FAMILY--RAJIDÆ.

_Disk broad, rhombic, generally with asperities or spines; tail with a longitudinal fold on each side. The pectoral fins extend to the snout. No electric organ; no serrated caudal spine._

RAJA.--Two dorsal fins on the tail, without spine; tail with
a rudimentary caudal fin, or without caudal. Each ventral fin
divided into two by a deep notch. Teeth small, obtuse, or
pointed. Pectoral fins not extending forwards to the extremity
of the snout. Nasal valves separated in the middle, where they
are without a free margin (see Fig. 1, p. 34).

Of all the genera of _Batoidei_, Rays have the widest geographical range; they are chiefly inhabitants of temperate seas, and much more numerous in those of the Northern than of the Southern Hemisphere. They advance more closely to the Arctic and Antarctic circles than any other member of this group. More than thirty species are known, of which the following are found on the British coast:--The Thornback (_R. clavata_), the Homelyn Ray (_R. maculata_), the Starry Ray (_R. radiata_), the Sandy Ray (_R. circularis_), the common Skate (_R. batis_), the Burton Skate (_R. marginata_), and the Shagreen Skate (_R. fullonica_). Some of these species, especially the Skates, attain a considerable size, the disk measuring six and even seven feet across. All are eatable, and some of them regularly brought to market. In the majority of the species peculiar sexual differences have been observed. In some, as in the Thornback, all or some of the teeth are pointed in the male sex, whilst they are obtuse and flat in the female. The males of all are armed with patches of claw-like spines, retractile in grooves of the integument, and serially arranged occupying a space on the upper side of the pectoral fin near the angle of the disk, and frequently also the sides of the head. In species which are armed with bucklers or asperities it is the female which is principally provided with these dermal productions, the male being entirely or nearly smooth. Also the colour is frequently different in the two sexes.

Other genera of this family are _Psammobatis_, _Sympterygia_, and _Platyrhina_. Although probably this family was well represented in cretaceous and tertiary formations, the remains found hitherto are comparatively few. _Arthropterus_, from the Lias, seems to have been a true Ray; and dermal spines of a species allied to the Thornback (_Raja antiqua_) are abundant in the crag deposits of Suffolk and Norfolk.

FIFTH FAMILY--TRYGONIDÆ.

_The pectoral fins are uninterruptedly continued to, and confluent at, the extremity of the snout. Tail long and slender, without lateral longitudinal folds; vertical fins none, or imperfectly developed, often replaced by a strong serrated spine._

The “Sting-Rays” are as numerous as the Rays proper, but they inhabit rather tropical than temperate seas. The species armed with a spine use it as a weapon of defence, and the wounds inflicted by it are, to man, extremely painful, and have frequently occasioned the loss of a limb. We have mentioned above (p. 190) that the danger arises from the lacerated nature of the wound as well as from the poisonous property of the mucus inoculated. The spines (Fig. 98, p. 190) are always barbed on the sides, and may be eight or nine inches long in the larger species. They are shed from time to time, and replaced by others growing behind the one in function, as the teeth of the fishes of this order, or as the fangs of a poisonous snake. Fossil species of _Trygon_ and _Urolophus_ occur in the tertiary strata of Monte Bolca and Monte Postale. The genera into which the various species have been divided are the following:--

UROGYMNUS.--Tail long, without fin or spine, sometimes with a
narrow cutaneous fold below. Body densely covered with osseous
tubercles. Teeth flattened.

Only one species is known (_U. asperrimus_), common in the Indian Ocean, and with a body from 4 to 5 feet long; the skin is frequently used for covering shields and the handles of swords and other weapons, its rough surface offering a firm hold to the hand.

TRYGON.--Tail very long, tapering, armed with a long
arrow-shaped barbed spine. Body smooth or with tubercles. Nasal
valves coalescent into a quadrangular flap. Teeth flattened.

Some twenty-five species are known, one of which (_T. pastinaca_) extends from the south coast of England and the east coast of North America through the Atlantic and Indian Ocean to Japan. The majority of the species belong to the tropical parts of the Indian and Atlantic Oceans; some inhabit exclusively freshwaters of eastern tropical America. A closely allied genus is _Tæniura_, with six species.

UROLOPHUS.--Tail of moderate length, with a distinct rayed
terminal fin, armed with a barbed spine, without or with a
rudimentary dorsal fin. Teeth flattened.

Seven species from tropical seas, apparently of small size.

PTEROPLATEA.--Body at least twice as broad as long; tail very
short and thin, without or with a rudimentary fin, and with a
serrated spine. Teeth very small, uni- or tri-cuspid.

Six species from temperate and tropical seas.

SIXTH FAMILY--MYLIOBATIDÆ.

_The disk is very broad, in consequence of the great development of the pectoral fins, which, however, leave the sides of the head free, and reappear at the extremity of the snout as a pair of detached (cephalic) fins. Viviparous._

“Devil-fishes,” “Sea-devils,” or “Eagle-rays.” Generally of large size, inhabiting temperate and tropical seas. Some genera possess a pair of singular cephalic processes, which generally project in a direction parallel to the longitudinal axis of the body, but are said to be flexible in the living fish, and used for scooping food from the bottom and conveying it to the mouth. In all the species the dentition consists of perfectly flat molars, forming a kind of mosaic pavement in both the upper and lower jaws: a most perfect mechanical arrangement for crushing alimentary substances.

MYLIOBATIS.--Teeth sexangular, large, flat, tessellated, those
in the middle much broader than long; several narrower series
on each side. Tail very long and thin, with a dorsal fin near
its root; generally a serrated spine behind the fin.

Seven species are known, two of which are European, one (_M. aquila_) being almost cosmopolitan, and occasionally found on the British coast. The young differ much from the adult, having no median series of larger teeth, but all the teeth of equal size and regularly sexangular. Also the tail is much longer in young examples than in old ones, and the coloration more ornamental. Teeth of species very closely allied to, or perhaps even identical with, existing species, are found in tertiary formations.

AËTOBATIS.--Form of the head, body, and tail as _Myliobatis_.
The nasal valves remain separate, each forming a long flap.
The lower dental lamina projects beyond the upper. Teeth flat,
broad, forming a single series, equivalent to the median series
of _Myliobatis_, there being no small lateral teeth.

One species only (_A. narinari_) which is found in almost all tropical seas, and of exceedingly common occurrence in the Atlantic and Indian Oceans; it does not seem to grow to a very large size (perhaps not exceeding 5 feet in width), and is readily recognised by numerous round bluish-white spots, with which the back is ornamented. Fossils of this genus occur in the English Eocenes and the Swiss Molasse.

RHINOPTERA.--The cephalic appendages are bent inwards, and situated at the lower side of the snout. Nasal valves confluent into a broad flap, with free margin. Teeth broad, flat, tessellated, in five or more series, the middle being the broadest, and the others decreasing in width outwards. Tail very slender, with a dorsal fin before the serrated spine.

Seven species from tropical and sub-tropical seas are known; of _Rhinoptera polyodon_ nothing is known except the jaws; and as its dentition is very peculiar, no opportunity should be lost of obtaining and preserving entire animals. Teeth very similar to those of existing species, and described as _Zygobatis_, occur in the Norwich Crag and in Miocene formations of Switzerland.

DICEROBATIS (CEPHALOPTERA).--Cephalic appendages pointing
straight forwards or inwards. Nostrils widely separated from
each other. Mouth inferior, wide. Both jaws with very numerous
and very small flat or tubercular teeth. Tail very slender,
with a dorsal fin between the ventrals, and with or without a
serrated spine.

CERATOPTERA.--Cephalic appendages pointing forwards or inwards.
Mouth anterior; wide. Teeth in the lower jaw only, very small.
Tail very slender, with a dorsal fin between the ventrals and
without spine.

The species of these two last genera are not yet well distinguished; about five of _Dicerobatis_ and two of _Ceratoptera_ are known from tropical and temperate seas, but their occurrence in the latter is rather sporadic. Some of them, if not all, attain an enormous size. One mentioned by Risso, taken off Messina, weighed 1250 pounds. Several observers speak of having seen them in pairs, the male being usually the smaller. Of a pair mentioned by Risso the female was first taken, and the male remained hovering about the boat for three days, and was afterwards found floating dead on the surface. Still larger individuals, but of uncertain species, are mentioned by Lacépède, who says that one taken at Barbadoes required seven yoke of oxen to draw it. A sketch of another, which was said to be twenty feet long, was sent to Lacépède; and Sonnini speaks of one which appeared to him to be longer and wider than the ship in which he was sailing. A fœtus taken from the uterus of the mother captured at Jamaica, and preserved in the British Museum, is five feet broad, and weighed twenty pounds. The mother measured fifteen feet in width as well as in length, and was between three and four feet thick. The capture of “Devil-fishes” of such large size is attended with danger, as they not rarely attack and capsize the boat. They are said to be especially dangerous when they accompany their young, of which they bring forth one only at a time.

SECOND SUB-ORDER--HOLOCEPHALA.

_One external gill-opening only, covered by a fold of the skin, which encloses a rudimentary cartilaginous gill-cover; four branchial clefts within the gill-cavity. The maxillary and palatal apparatus coalescent with the skull._

This suborder is represented in the living fauna by one family only, _Chimæridæ_; it forms a passage to the following order of fishes, the Ganoids. In external appearance, and with regard to the structure of their organs of propagation, the Chimæras are Sharks (See Fig. 96, p. 184). The males are provided with “claspers” in connection with the ventral fins, and the ova are large, encased in a horny capsule, and few in number; and there is no doubt that they are impregnated within the oviduct, as in Sharks. Chimæras are naked, but, as in _Scylliidæ_, very young individuals possess a series of small “placoid” spines, which occupy the median line of the back, and remind us of similar dermal productions in the Rays. The males, besides, are provided with a singular erectile appendage, spiny at its extremity, and received in a groove on the top of the head. On the other hand, the relations of the Chimæras to the Ganoid, and, more especially, Dipnoous type become manifest in their notochordal skeleton and continuity of cranial cartilage. The spine in front of the first dorsal fin is articulated to the neural apophysis, and not merely implanted in the soft parts, and immovable as in Sharks. A cartilaginous operculum makes its appearance, and the external gill-opening is single. The dentition is that of a Dipnoid, each “jaw” being armed with a pair of broad dental plates, with the addition of a pair of smaller cutting teeth in the upper “jaw.” Fossils of similar dental combination are not rare in strata, commencing from the Lias and the bottom of the Oolitic series; but it is impossible to decide in every case whether the fossil should be referred to the Holocephalous or Dipnoous type. According to Newberry, Chimæroid fishes commence in the Devonian with _Rhynchodus_, the remains of which were discovered by him in Devonian rocks of Ohio. Undoubted Chimæroids are _Elasmodus_, _Psaliodus_, _Ganodus_, _Ischyodus_, _Edaphodon_, and _Elasmognathus_, principally from mesozoic and tertiary formations. Very similar fossils occur in the corresponding strata of North America. A single species of _Callorhynchus_ has been discovered by H. Hector in the Lower Greensand of New Zealand.

The living Chimæras are few in number, and remain within very moderate dimensions, probably not exceeding a length of five feet, inclusive of their long filamentous, diphycercal tail. They are referred to two genera.

CHIMÆRA.--Snout soft, prominent, without appendage. The dorsal
fins occupying the greater part of the back, anterior with
a very strong and long spine. Longitudinal axis of the tail
nearly the same as that of the trunk, its extremity being
provided with a low fin above and below, similar in form to a
dorsal and anal fin. Anal fin very low.

Three species are known: _Ch. monstrosa_, from the coasts of Europe and Japan and the Cape of Good Hope; _Ch. colliei_ from the west coast of North America; and _Ch. affinis_ from the coast of Portugal. (See Fig. 96, p. 184.)

CALLORHYNCHUS.--Snout with a cartilaginous prominence,
terminating in a cutaneous flap. Two dorsal fins, the anterior
with a very strong and long spine. Extremity of the tail
distinctly turned upwards, with a fin along its lower edge, but
without one above. Anal fin close to the caudal, short and deep.

One species (_C. antarcticus_) is common in the Southern temperate zone. Cunningham describes the egg (see Fig. 81, p. 169), as being of a dark greenish-black colour, and, in general, measuring from eight to nine or even ten inches in length, by about three in breadth. It consists of a central, somewhat spindle-shaped convex area (between the horny walls of which the young fish lies), surrounded by a broad plicated margin, which is fringed at the edge, and covered on the under surface with fine light brownish-yellow hairs.

SECOND ORDER--GANOIDEI.

_Skeleton cartilaginous or ossified. Body with medial and paired fins, the hinder pair abdominal. Gills free, rarely partially attached to the walls of the gill-cavity. One external gill-opening only on each side; a gill-cover. Air-bladder with a pneumatic duct. Ova small, impregnated after exclusion. Embryo sometimes with external gills._

To this order belong the majority of the fossil fish remains of palæozoic and mesozoic age, whilst it is very scantily represented in the recent fauna, and evidently verging towards total extinction. The knowledge of the fossil forms, based on mere fragments of the hard parts of the body only, is very incomplete, and therefore their classification is in a most unsatisfactory state. In the following pages only the most important groups will be mentioned.

[For a study of details we have to refer to _Agassiz_, “Poissons
Fossiles;” _Owen_, “Palæontology,” Edinb. 1861, 8vo; _Huxley_,
“Preliminary Essay upon the Systematic Arrangement of the Fishes
of the Devonian Epoch,” in Mem. Geolog. Survey, Dec. 10; Lond.
1861, and “Illustrations of the Structure of Crossopterygian
Ganoids,” _ibid._ December 12, 1866; _Traquair_, “The Ganoids of
the British Carboniferous Formations,” part I. “Palæoniscidæ.”
Palæontogr. Soc. Lond. 1877.]

Eight suborders may be distinguished at present.

FIRST SUB-ORDER--PLACODERMI.

_Extinct. The head and pectoral region of the body encased in great bony, sculptured plates, with dots of enamel; the remainder of the body naked, or with ganoid scales; skeleton notochordal._

Comprises the oldest vertebrate remains, from Devonian and Carboniferous formations. _Pterichthys_: (Figs. 135 and 136), tail tapering, covered with small ganoid scales, without caudal fin; the cephalic shield was probably moveably joined to the cuirass of the trunk, and both were composed of several pieces; the abdominal shield consisted of one single median plate, and two pairs of lateral plates, a third small pair being sometimes observed detached in front of the anterior pair; pectoral exceedingly long, consisting of two pieces movably connected with each other; tail scaly, and short; a small dorsal fin placed on the tail; a pair of small ventrals; jaws small, with confluent denticles. Several species have been distinguished in remains found in the strata of Caithness and other localities in Scotland. _Coccosteus_ (Fig. 137, p. 354): all the bony plates are firmly united, no pectoral spines; tail naked and long; a dorsal and anal fin supported by interneural and interhæmal spines. Dentition unknown. _Dinichthys_: a gigantic fish from the Devonian of North America (estimated at from 15 to 18 feet in length), with the dermal covering very similar to that of Coccosteus, but with a simple arched dorsal shield. As in this latter genus the caudal extremity does not possess external or internal bony parts, and the ventral plastron of both genera corresponds in every particular; the dentition is so singularly like that of Lepidosiren, that Newberry (Geolog. Survey of Ohio, vol. ii. part 2) considers this genus to be in genetic relation to the Dipnoi. The following genera have been united in a separate family, _Cephalaspidæ_; viz. _Cephal__aspis_: head covered by a continuous shield with tubercular surface, produced into a horn at each posterior corner; a median dorsal backward prolongation bears a spine; heterocercal. _Auchenaspis_ and _Didymaspis_: allied to the preceding, but with the cephalic shield divided into a larger anterior and smaller posterior piece. _Pteraspis_: with the cephalic shield finely striated or grooved, composed of seven pieces. _Scaphaspis_ and _Cyathaspis_: with the surface of the head-shield similarly sculptured as in _Pteraspis_, but simple in the former, and composed of four pieces in the latter. _Astrolepis_: attained to the gigantic size of between twenty and thirty feet; its mouth was furnished with two rows of teeth, of which the outer ones were small, the inner much larger.

[See Ray Lankester, A. Monograph of the Fishes of the Old Red
Sandstone of Britain. Part I. Cephalaspidæ. Lond. 1868 and 1870.
4to.]

SECOND SUB-ORDER--ACANTHODINI.

Extinct. _Body oblong, compressed, covered with shagreen; skull not ossified; caudal heterocercal. Large spines, similar to those of Chondropterygians, in front of some of the median and paired fins. The spines are imbedded between the muscles, and not provided with a proximal joint._

_Acanthodes_, _Chiracanthus_, from Devonian and Carboniferous formations.

THIRD SUB-ORDER--DIPNOI.

_Nostrils two pairs, more or less within the mouth; limbs with an axial skeleton. Lungs and gills. Skeleton notochordal. No branchiostegals._[38]

FIRST FAMILY--SIRENIDÆ.

_Caudal fin diphycercal; no gular plates; scales cycloid. A pair of molars, above and below, and one pair of vomerine teeth._

LEPIDOSIREN.--Body eel-shaped, with one continuous vertical fin.
Limbs reduced to cylindrical filaments, without fringe. Vomerine
teeth conical, pointed. Each dental lamina or molar with strong
cusps, supported by vertical ridges. No external branchial
appendages; five branchial arches, with four intervening clefts.
Conus arteriosus with two longitudinal valves. Ovaries closed
sacs.

One species only is known from the system of the River Amazons (_L. paradoxa_). It must be very locally distributed, as but a few specimens have been brought to Europe, and all recent endeavours to obtain others have been unsuccessful. Natterer, by whom this most interesting fish was discovered, states that he obtained two specimens, one on the Madeira River, near Borba; the other in a backwater of the Amazons, above Villa Nova. The natives of the former place called it Carámurú, and considered it very scarce. The larger individual was nearly four feet long. It is said to produce a sound not unlike that of a cat, and to feed on the roots of mandioca and other vegetables. But, to judge from the dentition, this fish is much more likely to be carnivorous, like the following. It is one of the greatest desiderata of Natural History Collections.

[Natterer, “Annalen des Wiener Museum’s,” 1839, ii.; Bischoff.
“Annales des Sciences Naturelles,” 1840. xiv.]

PROTOPTERUS.--Very similar in the general form of the body and
dentition to _Lepidosiren_. Pectoral and ventral filaments with
a fringe containing rays. Three small branchial appendages
above the small gill-opening; six branchial arches, with five
intervening clefts. Conus arteriosus with two longitudinal
valves. Ovaries closed sacs.

_Protopterus annectens_ is the “Lepidosiren” which is commonly found in Zoological collections. It is usually imported from the west coast of Africa, where it abounds in many localities; but it is spread over the whole of tropical Africa, and forms in many districts of the central parts a regular article of food.

During the dry season, specimens living in shallow waters which periodically dry up, form a cavity in the mud, the inside of which they line with a protecting capsule of mucus, and from which they emerge again when the rains refill the pools inhabited by them. Whilst they remain in this torpid state of existence, the clay-balls containing them are frequently dug out, and if the capsules are not broken, the fishes imbedded in them can be transported to Europe, and released by being immersed in slightly tepid water. _Protopterus_ is exclusively carnivorous, feeding on water-insects, frogs, and fishes, and attains a length of six feet.

[Owen, “Trans. Linn. Soc.” 1841, xviii.]

CERATODUS.--Body elongate, compressed, with one continuous
vertical fin. Limbs paddle-shaped, with broad, rayed fringe.
Vomerine teeth incisor-like; molars with flat, undulated
surface, and lateral prongs. No external branchial appendages.
Conus arteriosus with transverse series of valves. Ovaries
transversely lamellated.[39]

Two species, _C. forsteri_ and _C. miolepis_, are known from fresh waters of Queensland. The specimens hitherto obtained have come from the Burnett, Dawson, and Mary rivers, some from the fresh waters of the upper parts, others from the lower brackish portions. The fish is said to attain to a weight of twenty pounds and to a length of 6 feet. Locally, the settlers call it “Flat-head,” “Burnett- or Dawson-Salmon,” and the aborigines “Barramunda,” a name which they appear to apply also to other large-scaled freshwater fishes, as the Osteoglossum leichardti. In the stomach there is generally found an enormous quantity of the leaves of plants growing on the banks of rivers, evidently eaten after they had fallen into the water and when in a decomposing condition. The flesh of the fish is salmon-coloured, and much esteemed as food.

The Barramunda is said to be in the habit of going on land, or at least on mud-flats; and this assertion appears to be borne out by the fact that it is provided with a lung. However, it is much more probable that it rises now and then to the surface of the water in order to fill its lung with air, and then descends again until the air is so much deoxygenised as to render a renewal of it necessary. It is also said to make a grunting noise, which may be heard at night for some distance. This noise is probably produced by the passage of the air through the œsophagus when it is expelled for the purpose of renewal. As the Barramunda has perfectly developed gills, beside the lung, we can hardly doubt that, when it is in water of normal composition, and sufficiently pure to yield the necessary supply of oxygen, these organs are sufficient for the purpose of breathing, and that the respiratory function rests with them alone. But when the fish is compelled to sojourn in thick muddy water charged with gases, which are the products of decomposing organic matter (and this must be the case very frequently during the droughts which annually exhaust the creeks of tropical Australia), it commences to breathe air with its lung in the way indicated above. If the medium in which it happens to be is perfectly unfit for breathing the gills cease to have any function; if only in a less degree the gills may still continue to assist in respiration. The Barramunda, in fact, can breathe by either gills or lungs alone, or by both simultaneously. It is not probable that it lives freely out of the water, its limbs being much too flexible for supporting the heavy and unwieldy body, and too feeble generally to be of much use in locomotion on land. However, it is quite possible that it is occasionally compelled to leave the water, although we cannot believe that it can exist without it in a lively condition for any length of time.

Of its propagation or development we know nothing, except that it deposits a great number of eggs of the size of those of a newt, and enveloped in a gelatinous case. We may infer that the young are provided with external gills, as in Protopterus and Polypterus.

The discovery of _Ceratodus_ does not date farther back than the year 1870, and proved to be of the greatest interest, not only on account of the relation of this creature to the other living _Dipnoi_ and _Ganoidei_, but also because it threw fresh light on those singular fossil teeth which are found in strata of Triassic and Jurassic formations in various parts of Europe, India, and America. These teeth, of which there is a great variety with regard to general shape and size, are sometimes two inches long, much longer than broad, depressed, with a flat or slightly undulated, always punctated crown, with one margin convex, and with from three to seven prongs projecting on the opposite margin.

SECOND FAMILY--CTENODODIPTERIDÆ.

_Caudal fin heterocercal. Gular plates. Scales cycloid. Two pairs of molars and one pair of vomerine teeth._

Extinct. _Dipterus_ (_Ctenodus_), _Heliodus_ from Devonian strata.

THIRD FAMILY--PHANEROPLEURIDÆ.

_Caudal fin diphycercal; vertical fin continuous. Gular plates. Scales cycloid. Jaws with a series of minute conical teeth on the margin._

Extinct. _Phaneropleuron_ from Devonian formations, and the carboniferous _Uronemus_ are probably generically identical.

FOURTH SUB-ORDER--CHONDROSTEI.

_Skeleton notochordal; skull cartilaginous, with dermal ossifications; branchiostegals few in number or absent. Teeth minute or absent. Integuments naked or with bucklers. Caudal fin heterocercal, with fulcra. Nostrils double, in front of the eyes._

FIRST FAMILY--ACIPENSERIDÆ.

_Body elongate, sub-cylindrical, with five rows of osseous bucklers. Snout produced, subspatulate or conical, with the mouth at its lower surface, small, transverse, protractile, toothless. Four barbels in a transverse series on the lower side of the snout. Vertical fins with a single series of fulcra in front. Dorsal and anal fins approximate to the caudal. Gill-membranes confluent at the throat and attached to the isthmus. Branchiostegals none. Gills four; two accessory gills. Air-bladder large, simple, communicating with the dorsal wall of the œsophagus._

Sturgeons are, perhaps, the geologically youngest Ganoids, evidence of their existence not having been met with hitherto in formations of older date than the Eocene clay of Sheppey. They are exclusively inhabitants of the temperate zone of the Northern Hemisphere, being either entirely confined to fresh water, or passing, for the purpose of spawning, a part of the year in rivers. They grow to a large size, and are the largest fishes of the fresh waters of the Northern Hemisphere, specimens 10 feet long being of common occurrence. The ova are very small, and so numerous that one female has been calculated to produce about three millions at one season; therefore their propagation, as well as their growth, must be very rapid; and although in many rivers their number is annually considerably thinned by the systematic manner in which they are caught when they ascend the rivers in shoals from the sea, no diminution has been observed. Wherever they occur they prove to be most valuable on account of their wholesome flesh. In Russia, besides, two not unimportant articles of trade are obtained from them, viz. Caviare, which is prepared from their ovaries, and Isinglass, which is made from the inner coats of their air-bladder. True Sturgeons are divided into two genera, _Acipenser_ and _Scaphirhynchus_.

ACIPENSER.--The rows of osseous bucklers are not confluent
on the tail. Spiracles present. Caudal rays surrounding the
extremity of the tail.

About twenty different species of Sturgeons may be distinguished from European, Asiatic, and American rivers. The best known are the Sterlet (_A. ruthenus_) from Russian rivers, celebrated for the excellency of its flesh, but rarely exceeding a length of three feet; the Californian Short-snouted Sturgeon (_A. brachyrhynchus_); the Hausen (_A. huso_), from rivers, falling into the Black Sea and the Sea of Azow (rare in Mediterranean), sometimes 12 feet long, and yielding an inferior kind of isinglass; the Chinese Sturgeon (_A. sinensis_); the Common Sturgeon of the United States (_A. maculosus_), which sometimes crosses the Atlantic to the coasts of Great Britain; Güldenstædt’s Sturgeon (_A. güldenstædtii_), common in European and Asiatic rivers, which yields more than one-fourth of the caviare and isinglass exported from Russia; the Common Sturgeon of Western Europe (_A. sturio_), which attains to a length of 18 feet, and has established itself also on the coasts of Eastern North America.

SCAPHIRHYNCHUS.--Snout spatulate; posterior part of the tail
attenuated and depressed, so that it is entirely enveloped by
the osseous scutes. Spiracles none. The caudal rays do not
extend to the extremity of the tail, which terminates in a
filament.

Four species are known: one (_S. platyrhynchus_) from the river-system of the Mississippi, and the three others from Central Asia; all are exclusively freshwater fishes; their occurrence in so widely distant rivers is one of the most striking instances by which the close affinity of the North American and North Asiatic faunas is proved.

SECOND FAMILY--POLYODONTIDÆ.

_Body naked, or with minute stellate ossifications. Mouth lateral, very wide, with minute teeth in both jaws. Barbels none. Caudal fin with fulcra. Dorsal and anal fins approximate to the caudal. Four gills and a half; no opercular gill or pseudobranchia._

POLYODON (SPATULARIA).--The snout is produced into an
exceedingly long, shovel-like process, thin and flexible on
the sides. Spiracles present. Gill-cover terminating in a very
long tapering flap. One broad branchiostegal. Each branchial
arch with a double series of very long, fine, and numerous
gill-rakers, the two series being divided by a broad membrane.
Air-bladder cellular. Upper caudal fulcra narrow, numerous.

The single species, _P. folium_, occurs in the Mississippi, and grows to a length of about six feet, of which the rostral shovel takes about one-fourth; in young examples it is comparatively still longer.

PSEPHURUS differs from _Polyodon_ in having the rostral
process less depressed and more conical. The gill-rakers are
comparatively short, in moderate number, and distant from one
another. Upper caudal fulcra enormously developed, and in small
number (six).

_Psephurus gladius_ inhabits the Yan-tse-Kiang and Hoangho, the distribution of the _Polyodontidæ_ being perfectly analogous to that of _Scaphirhynchus_. It grows to an immense size, specimens of 20 feet in length being mentioned by Basilewsky. The function of the rostral process in the economy of these fishes is not yet sufficiently explained. Martens believes that it serves as an organ of feeling, the water of those large Asiatic and American rivers being too turbid to admit of the Sturgeon seeing its prey, which consists of other fishes. The eyes of _Psephurus_, as well as _Polyodon_, are remarkably small. Both fishes are used as food.

Allied to the _Polyodontidæ_, and likewise provided with a paddle-shaped production of the fore part of the head, is the fossil genus _Chondrosteus_, remains of which occur in the Lias.

FIFTH SUB-ORDER--POLYPTEROIDEI.

_Paired fins with axial skeleton, fringed; dorsal fins two or more. Branchiostegals absent, but generally gular plates. Vertebral column diphycercal or heterocercal. Body scaly._

FIRST FAMILY--POLYPTERIDÆ.

_Scales ganoid; fins without fulcra. A series of dorsal spines, to each of which an articulated finlet is attached; anal placed close to the caudal fin, the vent being near the end of the tail. Abdominal portion of the vertebral column much longer than the caudal._

POLYPTERUS.--Teeth rasp-like, in broad bands in the jaws, on
the vomer and palatine bones; jaws with an outer series of
closely-set, larger, pointed teeth. Caudal fin surrounding the
extremity of the vertebral column; ventral fins well developed.
A spiracle on each side of the parietal, covered with an
osseous plate. A single large gular plate. Air-bladder double,
communicating with the ventral wall of the pharynx.

This Ganoid is confined to tropical Africa, occurring in abundance in the rivers of the west coast and in the Upper Nile; but it has not been found in the river-systems belonging to the Indian Ocean. It is scarce in the Middle and Lower Nile, and the specimens found below the Cataracts have been carried down, from southern latitudes, and do not propagate their species in that part of the river. There is only one species known, _Polypterus bichir_ (“Bichir” being its vernacular name in Egypt), which varies in the number of the dorsal finlets, the lowest being eight, the highest eighteen. It attains to a length of four feet. Nothing is known of its mode of life, and observations thereon are very desirable.

CALAMOICHTHYS.--Distinguished from _Polypterus_ by its greatly
elongate form, and the absence of ventral fins.

_C. calabaricus_, a dwarf form from Old Calabar.

SECOND FAMILY--SAURODIPTERIDÆ.

_Scales ganoid, smooth like the surface of the skull. Two dorsal fins; paired fins obtusely lobate. Teeth conical. Caudal heterocercal._

Extinct. _Diplopterus_, _Megalichthys_, and _Osteolepis_ from Devonian and Carboniferous formations.

THIRD FAMILY--COELACANTHIDÆ.

_Scales cycloid. Two dorsal fins, each supported by a single two-pronged interspinous bone; paired fins obtusely lobate. Air-bladder ossified; notochord persistent, diphycercal._

Extinct. _Coelacanthus_ from carboniferous strata; several other genera, from the coal formations to the chalk, have been associated with it--_Undina_, _Graphiurus_, _Macropoma_, _Holophagus_, _Hoplopygus_, _Rhizodus_.

FOURTH FAMILY--HOLOPTYCHIIDÆ.

_Scales cycloid or ganoid, sculptured. Two dorsal fins; pectorals narrow, acutely lobate; dentition dendrodont._

Extinct. In this family a peculiar type of dentition is found--the jaws are armed with two kinds of teeth, small ones serially arranged, and much larger fang-like teeth disposed at long intervals. Both kinds show at their base in transverse sections a labyrinthic complexity of structure, numerous fissures radiating from the central mass of vasodentine which fills up the pulp cavity, and sending off small ramifying branches. Genera belonging to this family are _Holoptychius_, _Saurichthys_, _Glyptolepis_, _Dendrodus_, _Glyptolaemus_, _Glyptopomus_, _Tristichopterus_, _Gyroptychius_, _Strepsodus_, from Devonian and Carboniferous strata.

SIXTH SUB-ORDER--PYCNODONTOIDEI.

_Body compressed, high and short or oval, covered with rhombic scales arranged in decussating pleurolepidal lines. Notochord persistent. Paired fins without axial skeleton. Teeth on the palate and hinder part of the lower jaw molar-like. Branchiostegals, but no gular plates._

Extinct. The regular lozenge-shaped pattern of the integuments of these fishes is described by Sir P. Egerton thus: “Each scale bears upon its inner anterior margin a thick solid bony rib, extending upwards beyond the margin of the scale, and sliced off obliquely, above and below, on opposite sides, for forming splices with the corresponding processes of the adjoining scales. These splices are so closely adjusted that, without a magnifying power or an accidental dislocation, they are not perceptible. When _in situ_, and seen internally, these continuous lines decussate with the true vertebral apophyses.” In some genera the “pleurolepidal” lines are confined to the anterior part of the side.

FIRST FAMILY--PLEUROLEPIDÆ.

_Homocercal. Body less high. Fins with fulcra._

_Pleurolepis_ and _Homoeolepis_ from the Lias.

SECOND FAMILY--PYCNODONTIDÆ.

_Homocercal. The neural arches and ribs are ossified; the roots of the ribs are but little expanded in the older genera, but enlarged in the tertiary forms, so as to simulate vertebræ. Paired fins not lobate. Obtuse teeth on the palate and the sides of the mandible; maxilla toothless; incisor-like teeth in the intermaxillary and front of the mandible. Fulcra absent in all the fins._

These fishes abound in Mesozoic and Tertiary formations. _Gyrodus_, _Mesturus_, _Microdon_, _Coelodus_, _Pycnodus_, _Mesodon_, are some of the genera distinguished by palæontologists. (See Fig. 102, p. 201.)

SEVENTH SUB-ORDER--LEPIDOSTEOIDEI.

_Scales ganoid, rhombic; fins generally with fulcra; paired fins not lobate. Præ- and inter-operculum developed; generally numerous branchiostegals, but no gular plate._

FIRST FAMILY--LEPIDOSTEIDÆ.

_Scales ganoid, lozenge-shaped. Skeleton completely ossified; vertebræ convex in front and concave behind. Fins with fulcra; dorsal and anal composed of articulated rays only, placed far backwards, close to the caudal. Abdominal part of the vertebral column much longer than caudal. Branchiostegals not numerous, without enamelled surface. Heterocercal._

_Lepidosteus._--Body elongate, sub-cylindrical; snout elongate,
spatulate, or beak-shaped; cleft of the mouth wide; both jaws
and palate armed with bands of rasp-like teeth and series
of larger conical teeth. Four gills; no spiracles; three
branchiostegals. Air-bladder cellular, communicating with the
pharynx.

Fishes of this genus existed already in Tertiary times; their remains have been found in Europe as well as North America. In our period they are limited to the temperate parts of North America, Central America, and Cuba. Three species can be distinguished which attain to a length of about six feet. They feed on other fishes, and their general resemblance to a pike has given to them the vernacular names of “Gar-Pike,” or “Bony Pike.”

SECOND FAMILY--SAURIDÆ.

_Body oblong, with ganoid scales; vertebræ not completely ossified; termination of the vertebral column homocercal; fins generally with fulcra. Maxillary composed of a single piece; jaws with a single series of conical pointed teeth. Branchiostegals numerous, enamelled, the anterior broad gular plates._

Extinct. Numerous genera occur in Mesozoic formations; one with the widest range is _Semionotus_, with distichous fulcra, from the Lias and Jura; _Eugnathus_, with large posteriorly serrated scales, and fulcra on nearly all fins; _Cephenoplosus_ from the Upper Lias; _Macrosemius_ from the Oolite; _Propterus_, _Ophiopsis_, _Pholidophorus_, _Pleuropholis_, _Pachycormus_, _Oxygnathus_, _Ptycholepis_, _Conodus_, _Eulepidotus_, _Lophiostomus_, etc.

THIRD FAMILY--STYLODONTIDÆ.

_Body rhombic or ovate, with ganoid scales; vertebræ not completely ossified; termination of the vertebral column homocercal; fins with fulcra. Maxillary composed of a single piece; jaws with several series of teeth, the outer ones equal, styliform. Dorsal fin very long, extending to the caudal. Branchiostegals numerous._

Extinct. _Tetragonolepis_ from the Lias (see Fig. 103, p. 207).

FOURTH FAMILY--SPHÆRODONTIDÆ.

_Body oblong, with rhombic ganoid scales; vertebræ ossified, but not completely closed; homocercal; fins with fulcra. Maxillary composed of a single piece; teeth in several series, obtuse; those on the palate globular. Dorsal and anal fins short. Branchiostegals._

Extinct. The type genus of this family is _Lepidotus_, so named from its large rhombic, dense, and polished scales. The dorsal is opposite to the anal, and all the fins are provided with a double row of fulcra. This genus ranges from the Lias to the Chalk; one species would seem to have survived into tertiary times, if it should not prove to be a _Lepidosteus_.

FIFTH FAMILY--ASPIDORHYNCHIDÆ.

_Body elongate, with ganoid scales; jaws prolonged into a beak; termination of the vertebral column homocercal. Fins with fulcra; a series of enlarged scales along the side of the body. Dorsal fin opposite to the anal._

Extinct; mesozoic. _Aspidorhynchus_ has the upper jaw longer than the lower; very peculiar is the occurrence of a single, solid, conical bone, situated in front of the symphysis of the lower jaw, to which it is joined by a suture. _Belonostomus_ with both jaws of equal length.

SIXTH FAMILY--PALÆONISCIDÆ.

_Body fusiform, with rhombic ganoid scales. Notochord persistent, with the vertebral arches ossified. Heterocercal. All the fins with fulcra; dorsal short. Branchiostegals numerous, the foremost pair forming broad gulars. Teeth small, conical, or cylindrical._

Extinct. Many genera are known; from the Old Red Sandstone--_Chirolepis_ and _Acrolepis_; from Carboniferous rocks--_Cosmoptychius_, _Elonichthys_, _Nematoptychius_, _Cycloptychius_, _Microconodus_, _Gonatodus_, _Rhadinichthys_, _Myriolepis_, _Urosthenes_; from the Permian--_Rhabdolepis_, _Palæoniscus_, _Amblypterus_ and _Pygopterus_; from the Lias--_Centrolepis_, _Oxygnathus_, _Cosmolepis_, and _Thrissonotus_.

[See Traquair, “The Ganoid Fishes of the British Carboniferous
Formations.” Part I. _Palæoniscidæ._]

SEVENTH FAMILY--PLATYSOMIDÆ.

_Body generally high, compressed, covered with rhombic ganoid scales arranged in dorso-ventral bands. Notochord persistent, with the vertebral arches ossified. Heterocercal; fins with fulcra; dorsal fin long, occupying the posterior half of the back. Branchiostegals numerous. Teeth tubercular or obtuse._

Extinct. From Carboniferous and Permian formations--_Eurynotus_, _Benedenius_, _Mesolepis_, _Eurysomus_, _Wardichthys_, _Chirodus_ (M’Coy), _Platysomus_.

[See Traquair, “On the Structure and Affinities of the Platysomidæ”, in
“Trans. Roy. Soc.,” Edinb., vol. xxix.]

EIGHTH SUB-ORDER--AMIOIDEI.

_Vertebral column more or less completely ossified, heterocercal. Body covered with cycloid scales. Branchiostegals present._

FIRST FAMILY--CATURIDÆ.