In front of the exoccipital is the large pro-otic pierced by two prominent foramina. Through the more dorsal of these (fig. 10, VII.) the facial nerve passes out, while the more ventral (fig. 10, 16) is for the passage of an artery. Dorsal to the exoccipital are the opisthotic and pterotic, and dorsal to the pro-otic is the sphenotic. The pterotic is marked by a prominent groove often lined by cartilage, which is continued forwards along a tract of cartilage between the pro-otic and sphenotic. With this groove the hyomandibular articulates.

There are considerable ossifications in the sphenoidal region of the side of the cranium. The anterior boundary of the posterior fontanelle is formed by the large alisphenoid, which is continuous behind with the pro-otic and sphenotic, and below with a slender basisphenoid. Both in front of and behind the basisphenoid there are considerable vacuities in the walls of the cranium; through the posterior of these openings (fig. 10, V.) the main part of the trigeminal nerve passes out, and through the anterior one, the optic (fig. 10, II.). The alisphenoid is continuous in front with the orbitosphenoid (fig. 10, 10), which is pierced by the foramen for the exit of the first nerve (fig. 10, I.), and in front of the orbitosphenoid there is a large vacuity. The lateral ethmoid is seen in the side view as well as in the dorsal view. Further forwards are seen the olfactory pits, and the long cartilaginous snout.

A ventral view of the cartilaginous cranium shows much the same points as the side view. The basisphenoid appears on the surface immediately in front of the basi-occipital.

The Skull with membrane bones.

The dorsal surface. The greater part of the dorsal surface in front of the supra-occipital is overlaid by a pair of large rough frontals (figs. 9, A, 5, and 10, 5). They cover the posterior fontanelles and stretch over from the sphenotic to the lateral ethmoid, forming a roof for the orbit. They meet in the middle line behind, but in front are separated by a narrow tract of unossified cartilage, and are overlapped by the median ethmoid (figs. 9, A, 6, and 11, 6). At the sides of the supra-occipital behind the frontals are a pair of small parietals (figs. 9, A, 7, and 11, 7).

In a ventral view the cranium is seen to be chiefly covered by two large membrane bones, the parasphenoid (fig. 9, B, 9) behind, the vomer in front. A view of the posterior end differs from that of the cartilaginous cranium only in the fact that the end of the parasphenoid appears lying ventral to the basi-occipital.

The lateral view differs very markedly from that of the cartilaginous cranium, there being a great development of membrane bone in connection with the jaws and branchial apparatus. Lying dorsally are seen the median ethmoid, frontal, parietal, and supra-occipital as before. Lying external to the middle of the median ethmoid is seen the small nasal (fig. 11, 8), and below the hinder part is the lachrymal. The lachrymal (fig. 11, 9) forms the first of a series of seven small bones which surround the orbit forming the orbital ring. Of these the one lying immediately in the mid-ventral line of the orbit is the suborbital, while the one lying in the mid-dorsal line and attached to the frontal is the supra-orbital (fig. 11, 11). The orbit has a cartilaginous sclerotic in which are two small ossifications (fig. 11, 13) laterally placed.

Bones of the upper jaw.

The palato-pterygo-quadrate bar is in a very different condition from that of the dogfish, it is partially cartilaginous, partially converted into cartilage bone, partially overlapped by membrane bone. It is narrow in front but becomes much broader and deeper when followed back. Its anterior end forms the palatine which bears teeth, and in front is completely ossified, while behind the cartilage is only sheathed by bone.

Just behind the palatine the outer part of the cartilage is ossified, forming two small bones, the pterygoid and meso-pterygoid, while behind them is a larger, somewhat square bone, the meta-pterygoid (fig. 11, 15).

Below the meta-pterygoid is a tract of unossified cartilage, and then comes the quadrate (fig. 11, 18).

The lower angle of the quadrate bears a cartilaginous condyle with which the mandible articulates. In front of the palatine the cartilaginous snout is overlapped by three membrane bones, the jugal, maxilla and premaxillae.

The premaxillae (fig. 11, 20), the largest of these, overlaps the maxilla behind; both bones bear teeth. The jugal (fig. 11, 17) lies above the maxilla and overlaps it in front.

The lower jaw.

The lower jaw is a strong bar and is like the upper jaw, partly cartilaginous, forming Meckel's cartilage, partly ossified, and sheathed to a considerable extent in membrane bone.

The outer side and posterior end is ossified, forming the large articular (fig. 11, 21), but the condyle is cartilaginous and the anterior part of the articular forms merely a splint on the outer side of Meckel's cartilage, which extends beyond it for a considerable distance. The angle of the jaw just below the condyle is formed by a small angular (fig. 11, 22), and the anterior two-thirds of the jaw is sheathed in the large tooth-bearing dentary (fig. 11, 23).

The Hyoid arch.

The hyoid arch has a number of ossifications in it and is closely connected with the mandibular arch.

The hyomandibular (fig. 11, 24) is a large bone which articulates with a shallow groove lined by cartilage and formed partly in the pterotic, partly in front of it. The hyomandibular is overlapped in front by the meta-pterygoid, while below it tapers and is succeeded by a small area of unossified cartilage followed by the forwardly-directed symplectic which fits into a groove in the quadrate.

The unossified tract between the hyomandibular and symplectic is continuous in front with a strong bar, which remains partly cartilaginous and is partly converted into cartilage bone. The proximal part is ossified, forming the epi-hyal, the middle part forms the cerato-hyal (fig. 11, 27), in front of which is the small hypo-hyal. The hyoid arches of the two sides are united by the large tooth-bearing glosso-hyal (fig. 11, 29). Attached to the lower surface of the hyoid arch are a series of twelve flat branchiostegal rays (fig. 11, 35). Each overlaps the one in front of it, the posterior one being the largest. The branchiostegal rays of the two sides are united in front by an unpaired membrane bone, the basi-branchiostegal (fig. 11, 36).

Opercular bones. Behind the hyomandibular there is a large bony plate, the operculum, formed of four large membrane bones. The anterior of these, the pre-opercular (fig. 11, 33), is crescentic in shape, and with its upper end a small supratemporal (fig. 11, 34) is connected.

Behind the upper part of the pre-opercular is the largest of the opercular bones, the opercular proper. Its lower edge overlaps the sub-opercular, and both opercular and sub-opercular are overlapped by the infra-opercular (fig. 11, 32) in front. The infra-opercular is in its turn overlapped by the pre-opercular.

Branchial arches.

There are five branchial arches, the first four of which bear gill rays. Each of the first three consists of a shorter upper portion directed obliquely backwards and outwards, and a longer lower portion forming a right angle with the upper and directed obliquely forwards and inwards. The greater part of each arch is ossified.

The upper part of either of the first two consists of a short tapering pharyngo-branchial directed inwards, and of a long epi-branchial tipped with cartilage at both ends. The junction of the upper and lower parts is formed by a cartilaginous hinge-joint between the epi-branchial and cerato-branchial. The cerato-branchial is a long bony rod separated by a short area of cartilage from the hypo-branchial, which is succeeded by the basibranchial meeting its fellow in the middle line. The fourth arch has a short epi-branchial and no ossified pharyngo-branchial, while the fifth is reduced to little more than the cerato-branchial, which bears a few teeth on its inner edge. All the branchial arches have projecting from their surfaces a number of little processes which act as strainers. The first and fourth arches have one series of these, the second and third have two.

THE SKULL OF THE CODFISH[37].

A full description having been already given of the Salmon's skull, that of the Codfish will be described in a briefer manner. The skull is very fully ossified, and the great number of plate-like bones render it a very complicated structure.

The Cranium.

At the posterior end of the dorsal surface is the large supra-occipital, which is drawn out behind into the large blade-like occipital spine. On each side of the supra-occipital are the small irregular parietals, while in front of it the roof of the skull is mainly formed by the very large unpaired frontal.

A complicated series of bones are developed in connection with the auditory capsule, which forms a large projecting mass united with the side of the cranium and drawn out behind into a pair of strong processes, the epi-otic and parotic processes. Both these processes are connected behind with a large V-shaped bone, the post-temporal (fig. 13, 1), which will be described when dealing with the pectoral girdle. The epi-otic process is formed by the epi-otic, which is continuous in front with the parietal. The parotic process is formed by two larger bones, a more dorsal one, the pterotic, and a more ventral and internal one, the opisthotic, which is continuous in front with the large pro-otic. Intervening between the pterotic and frontal is another rather large bone, the sphenotic, this articulates below with the pro-otic. The pterotic and sphenotic together give rise to a large concave surface by which the hyomandibular articulates with the cranium. Several of the cranial nerves pass out through the bones of the auditory capsule. The ninth leaves by a foramen near the posterior border of the opisthotic, the fifth and seventh by a notch in the anterior border of the pro-otic.

A number of bones are likewise developed in connection with the orbit forming the orbital ring. Of these the most anterior, the lachrymal, is much the largest, the others are five to seven in number, the most ventral being the suborbital. The sclerotic coat of the eye is cartilaginous.

Two pairs of bones and one unpaired bone are developed in connection with the olfactory capsules, of these, the nasals are narrow bones lying next the lachrymals, but nearer the middle line; they overlap the second pair of bones, the irregular lateral ethmoids. These meet one another in the middle line, and are overlapped behind by the frontal. They articulate laterally with the lachrymal and palatine, and ventrally with the parasphenoid.

In a posterior view the foramen magnum and the four bones which surround it and together form the occipital segment are well seen. On the ventral side is the basi-occipital, terminated posteriorly by a slightly concave surface which articulates with the centrum of the first vertebra. The sides of the foramen magnum are formed by the exoccipitals, a pair of very irregular bones, pierced by a pair of prominent foramina for the exit of the tenth nerves. The exoccipitals also bear a pair of surfaces for articulation with corresponding ones on the neural arch of the first vertebra. The most dorsal of the four bones is the supra-occipital.

On the ventral surface of the cranium in front of the basi-occipital is seen the parasphenoid, a very long narrow bone which underlies the greater part of the cranium. Behind, it articulates dorsally with the basi-occipital and dorsolaterally with the pro-otics and opisthotics, in front it articulates dorsally with the lateral ethmoid and ventrally with the vomer. At the sides of the parasphenoid are the small alisphenoids articulating above with the postfrontals, in front with the frontals, and behind with the pro-otics.

The vomer is an unpaired bone lying immediately in front of the parasphenoid. In front it terminates with a thickened curved margin bearing several rows of small teeth; behind it tapers out into a long process which underlies the anterior part of the parasphenoid. Immediately dorsal to the vomer is another median bone, the median ethmoid; this is truncated in front and tapers out behind into a process which fits into a groove on the ventral side of the frontal.

Bones in connection with the upper jaw.

These bear a close resemblance to those of the Salmon. The most anterior bone is the premaxillae, a thick curved bone meeting its fellow in the middle line. The point of junction of the two is drawn out into a short process, and the oral surface is thickly covered with small teeth. The dorsal ends of the premaxillae are seen in the fresh skull to meet a large patch of cartilage. Behind the premaxillae is the maxilla, a long rod-like toothless bone, somewhat expanded at the upper end where it articulates with the premaxillae and vomer.

Articulating in front with the anterior end of the maxilla and with the lateral ethmoid is a very irregular bone, the palatine (fig. 12, 1); it articulates behind with two flat bones, the pterygoid and meso-pterygoid. The pterygoid is united behind with two more bones, the quadrate (fig. 12, 4) and meta-pterygoid. The quadrate is a rather stout irregular bone, bearing on its lower surface a prominent saddle-shaped articulating surface for the mandible. The palatine, pterygoid and quadrate bones are the ossified representatives of the palato-pterygo-quadrate bar of the Dogfish.

The quadrate is united behind with the symplectic (fig. 12, 5), and the meta-pterygoid with the symplectic and hyomandibular, both of which bones will be described immediately in connection with the hyoid arch.

The Lower jaw.

The lower jaw or mandible like that of the Salmon is partly cartilaginous, forming Meckel's cartilage, partly formed of cartilage bone, partly of membrane bone. Meckel's cartilage is of course not seen in the dried skull.

The lower jaw includes one cartilage bone, the articular (fig. 12, 9), this is a large bone connected by a saddle-shaped surface with the quadrate. Meckel's cartilage lies in a groove on its under surface, and projects beyond it in front. The angular is a small thick bone united to the lower surface of the articular at its posterior end. The dentary (fig. 12, 10) is a large tooth-bearing bone meeting its fellow in the middle line in front, while the articular fits into a deep notch at its posterior end.

The hyoid arch.

The hyomandibular (fig. 12, 7) is a large irregular bone, articulating by a prominent rounded head with the sphenotic and pterotic. It is united in front with the meta-pterygoid and symplectic, and sends off behind a strong process which articulates with the opercular. The symplectic is a long somewhat triangular bone drawn out in front into a process which fits into a groove on the inner surface of the quadrate. The distal portion of the hyoid arch is strongly developed and consists of first the inter-hyal (fig. 12, 11), a short bony rod, which articulates dorsally with a patch of cartilage intervening between the posterior part of the hyomandibular and the symplectic. Below it is united with the apex of the triangular epi-hyal, a bone suturally connected with the large cerato-hyal (fig. 12, 13) which unites distally with two small hypo-hyals. To the cerato-hyal are attached a series of seven strong curved cylindrical rods, the branchiostegal rays. The first of these is the smallest and they increase in size up to the last. The four dorsal ones are attached to the outer surface of the cerato-hyal, the three ventral ones to its inner surface. Interposed between the hypo-hyals of the two sides is an unpaired somewhat triangular plate, the uro-hyal or basi-branchiostegal (fig. 12, 15).

The branchial arches.

The branchial arches are five in number and consist of the following parts on each side. The dorsal end is formed of the supra-pharyngeal bone, a large irregular bone covered ventrally with teeth of a fair size, and representing the fused pharyngo-branchials of the four anterior arches. Its external surface is continuous with four small epi-branchials which pass horizontally backwards and outwards. Their distal ends meet four long cerato-branchials which are directed forwards and inwards and form the principal part of the arches.

Each of the first three cerato-branchials articulates ventrally with a hypo-branchial, and the hypo-branchials of the two sides are united in the middle line by an unpaired basibranchial. The third hypo-branchial is much flattened. The fourth cerato-branchial is united by cartilage with the posterior surface of the third hypo-branchial, which it meets near the middle line.

The fifth arch consists only of the cerato-branchial, a wide structure covered with teeth and generally called the inferior pharyngeal bone.

The skeleton of the operculum consists of the same four bones as in the Salmon, namely the opercular, the infra-opercular, the pre-opercular and the sub-opercular. Of these the anterior bone, the pre-opercular, is the largest, while the infra-opercular is the smallest. The opercular has a facet for articulation with the hyomandibular.

2. The Appendicular Skeleton.

The Pectoral girdle.

This is of a highly specialised type. Membrane bones are greatly developed, and the cartilage bones, the scapula and coracoid, are much reduced in size and importance.

The largest bone in the shoulder girdle is the clavicle (fig. 13, 3), which is irregularly crescent shaped, thick in front and tapering off behind. To the outer side of its upper part is attached a thick cylindrical bone, the supra-clavicle, which passes upwards and is connected with a strong V shaped bone, the post-temporal. The apex of the V meets the supra-clavicle, the inner limb articulates with the epi-otic process, the outer with the parotic process. Projecting downwards from the upper part of the clavicle is a long bony rod, flattened proximally and cylindrical and pointed distally, this is the post-clavicle (fig. 13, 6).

The scapula (fig. 13, 5) is a small irregular plate of bone attached to the inner side of the middle of the clavicle. The coracoid[38] is a larger plate of similar character, irregularly triangular in shape, attached to the inner side of the clavicle immediately below the scapula. The scapula and coracoid bear the pectoral fin.

The Pectoral fins.

Each of these consists of four small irregular bones, the brachial ossicles (fig. 13, 7), bearing a series of about nineteen dermal fin-rays. The brachial ossicles represent the reduced and modified radiale and basalia of cartilaginous fish such as the dogfish. The fin-rays (fig. 13, 8) which form the whole external portion of the fin are long slender rods having essentially the same character as those of the unpaired fins.

The Pelvic girdle.

The pelvic girdle in the Cod as in other Teleosteans is entirely absent, its place being taken by the enlarged basi-pterygia of the fins.

The Pelvic fins.

These have a very anomalous position in the Cod, being attached to the throat in front of the pectoral girdle. Each consists of a basal portion, the basi-pterygium, and of a number of dermal rays. The basi-pterygium consists of an expanded ventral portion which meets its fellow below in the middle line, and to which the rays are attached, and of an inwardly-directed dorsal portion which also meets its fellow and is imbedded in the flesh. The rays are six in number and are long slender structures similar to those of the other fins.

CHAPTER VIII.
GENERAL ACCOUNT OF THE SKELETON IN FISHES[39].

EXOSKELETON.

The most primitive type of exoskeleton is that found in Elasmobranchs and formed of placoid scales; these are tooth-like structures consisting of dentine and bone capped with enamel, and have been already described (p. 4). In most Elasmobranchs they are small and their distribution is fairly uniform, but in the Thornback skate, Raia clavata, they have the form of larger, more scattered spines. In adult Holocephali and in Polyodon and Torpedo there is no exoskeleton, in young Holocephali, however, there are a few small dorsal ossifications.

The plates or scales of many Ganoids may have been formed by the gradual fusion of elements similar to these placoid scales, and often bear a number of little tooth-like processes. In Lepidosteus, Polypterus, and many extinct species, these ganoid scales, which are rhomboidal in form and united to one another by a peg and socket articulation, enclose the body in a complete armour. In Trissolepis part of the tail is covered by rhomboidal scales, while rounded scales cover the trunk and remainder of the tail. Acipenser and Scaphirhynchus have large dermal bony plates which are not rhomboidal in shape and do not cover the whole body. In Acipenser a single row extends along the middle of the back and two along each side.

The majority of Teleosteans have thin flattened scales which differ from those of Ganoids in being entirely mesodermal in origin, containing no enamel. There are two principal types of Teleostean scales, the cycloid and ctenoid. A cycloid scale is a flat thin scale with concentric markings and an entire posterior margin. A ctenoid scale differs in having its posterior margin pectinate. The Dipnoi have overlapping cycloid scales. The rounded scales of Amia and of many fossil ganoids such as Holoptychius are shaped like cycloid scales, but differ from them in being more or less coated with enamel. In Eels and some other Teleosteans the scales are completely degenerate and have almost disappeared. Some Teleosteans, like Diodon hystrix, have scales with triradiate roots from which arise long sharp spines directed backwards. These scales, which resemble teeth, contain no enamel; they become erect when the fish inflates its body into a globular form. Many Siluroids have dermal armour in the form of large bony plates which are confined to the anterior part of the body. In Ostracion the whole body is covered by hexagonal plates, closely united together.

The fin-rays are structures of dermal origin which entirely or partially support the unpaired fins, and assist the bony or cartilaginous endoskeleton in the support of the paired fins.

In Elasmobranchs, Dipnoi, and Chondrosteous ganoids the skeletons of the fins are, as a rule, about half of exoskeletal, half of endoskeletal origin, the proximal and inner portion being cartilaginous and endoskeletal, the distal and outer portion being exoskeletal, and consisting of horny or of more or less calcified fin-rays. In bony Ganoids and Teleosteans the endoskeletal parts are greatly reduced and the fins come to consist mainly of the fin-rays, which are ossified and frequently become flattened at their distal ends.

The fin-rays of the ventral part of the caudal fin are carried by the haemal arches; those of the dorsal and anal fins and of the dorsal part of the caudal fin generally by interspinous bones, which in adult Teleosteans alternate with the neural and haemal spines. In Dipnoi these interspinous bones articulate with the neural and haemal spines. In many Siluroids the anterior rays of the dorsal and pectoral fins are developed into large spines which often articulate with the endoskeleton, or are sometimes fused with the dermal armour plates. Similar spines may occur in Ganoids in front of both the dorsal and anal fins. Polypterus has a small spine or fulcrum in front of each segment of the dorsal fin. Such spines are often found fossilised, and are known as ichthyodorulites.

Similar spines are found in many Elasmobranchs, but they are simply inserted in the flesh, not articulated to the endoskeleton. They also differ from the spines of Teleosteans and Ganoids in the fact that they are covered with enamel, and often have their edges serrated like teeth. In the extinct Acanthodii they generally occur in front of all the fins, paired and unpaired.

In Trygon, the Sting-ray, the tail bears a serrated spine which is used for purposes of offence and defence. Many ichthyodorulites may have been spines of this nature fixed to the tail, rather than spines situated in front of the fins. The spines, which are always found in front of the dorsal fin in Holocephali, agree with those of Elasmobranchs in containing enamel, and with those of Teleosteans in being articulated to the endoskeleton.

Teeth.

The teeth of fish[40] are subject to a very large amount of variation, perhaps to more variation than are those of any other class of animals. Sometimes, as in adult Sturgeons, they are entirely absent, sometimes they are found on all the bones of the mouth, and also on the hyoid and branchial arches. The teeth are all originally developed in the mucous membrane of the mouth, but they afterwards generally become attached to firmer structures, especially to the jaws. In Elasmobranchs, however, they are generally simply imbedded in the tough fibrous integument of the mouth. Their attachment to the jaws may take place in three different ways.

(1) By an elastic hinge-joint, as in the Angler (Lophius), and the Pike (Esox lucius). In the Angler the tooth is held by a fibrous band attaching its posterior end to the subjacent bone, in the Pike by uncalcified elastic rods in the pulp cavity.

(2) By ankylosis, i.e. by the complete union of the calcified tooth substance with the subjacent bone. This is the commonest method among fish.

(3) By implantation in sockets. This method is not very common among fish. The teeth are sometimes, as in Lepidosteus, ankylosed to the base of the socket. In this genus there is along each ramus of the mandible a median row of large teeth placed in perfect sockets, and two irregular lateral rows of small teeth ankylosed to the jaw.

Dentine, enamel and cement are all represented in the teeth of fishes, but the enamel is generally very thin, and cement is but rarely developed. Dentine forms the main bulk of the teeth; it is sometimes of the normal type, but generally differs from that in higher vertebrates in being vascular, and is known as vasodentine. A third type occurs, known as osteodentine; it is traversed by canals occupied by marrow, and is closely allied to bone.

The teeth are generally continually renewed throughout life, but sometimes one set persists.

The teeth of Selachii are fundamentally identical with placoid scales. They are developed from a layer of dental germs which occurs all over the surface of the skin, except in the region of the lips. At this point the layer of tooth-producing germs extends back into the mouth, being projected by a fold of the mucous membrane (fig. 14, 7). Here new teeth are successively formed, and as they grow each is gradually brought into a position to take the place of its predecessor by the shifting outwards of the gum over the jaw. Owing to this arrangement sharks have practically an unlimited supply of teeth (figs. 14 and 15).

Two principal types of teeth are found in Elasmobranchs. In Sharks and Dogfish, on the one hand, the teeth are very numerous, simple, and sharp-pointed, and are with or without serrations and lateral cusps. Many Rays and fossil Elasmobranchs, on the other hand, have broad flattened teeth adapted for crushing shells. Intermediate conditions occur between these two extremes. Thus in Cestracion and many extinct sharks, such as Acrodus, while the median teeth are sharp, the lateral teeth are more or less flattened and adapted for crushing. In various species belonging to the genus Raia the teeth of the male are sharp, while those of the female are blunt. A very specialised dentition is met with in the Eagle-rays (Myliobatidae), in which the jaws are armed with flattened angular tooth-plates, arranged in seven rows, forming a compact pavement; the plates of the middle row are very wide and rectangular, those of the other rows are much smaller and hexagonal. Lastly, in Cochliodus the individual crushing teeth are fused, forming two pairs of spirally-coiled dental plates on each side of each jaw. Pristis, the Saw-fish, has a long flat cartilaginous snout, bearing a double row of persistently-growing teeth planted in sockets along its sides. Each tooth consists of a number of parallel dentinal columns, united at the base, but elsewhere distinct.

In the Holocephali—Chimaera, Hariotta and Callorhynchus—only three pairs of teeth or dental plates occur, two pairs in the upper jaw, one in the lower. These structures persist throughout life and grow continuously. The upper tooth structures are attached respectively to the ethmoid or vomerine region of the skull, and to the palato-pterygoids. The vomerine teeth are small, while those attached to the mandible and the palato-pterygoid region are large and bear several roughened ridges adapted for grinding food. The teeth of the two opposite sides of the jaw meet in a median symphysis. The teeth of Chimaera are more adapted for cutting, those of Callorhynchus for crushing. Many extinct forms are known, some of whose teeth are intermediate in structure between those of Chimaera and Callorhynchus.

The teeth of Ganoids are also extremely variable. Among living forms, the Holostei are more richly provided with teeth than are any other fishes, as they may occur on the premaxillae, maxillae, palatines, pterygoids, parasphenoid, vomers, dentaries, and splenials. Among the Chondrostei, on the other hand, the adult Acipenseridae are toothless; small teeth however occur in the larval sturgeon, and in Polyodon many small teeth are found attached merely to the mucous membrane of the jaws. Many fossil Ganoids have numerous flattened or knob-like teeth, borne on the maxillae, palatines, vomers and dentaries. Others have a distinctly heterodont dentition. Thus in Lepidotus the premaxillae bear chisel-like teeth, while knob-like teeth occur on the maxillae, palatines and vomers. In Rhizodus all the teeth are pointed, but while the majority are small a few very large ones are interspersed.

In Teleosteans, too, the teeth are eminently variable both in form and mode of arrangement. They may be simple and isolated, or compound, and may be borne on almost any of the bones bounding the mouth cavity, and also as in the Pike, on the hyoid and branchial arches. The splenial however never bears teeth and the pterygoid and parasphenoid only rarely, thus differing from the arrangement in the Holostei.

The isolated teeth are generally conical in form and are ankylosed to the bone that bears them. Such teeth are, with a few exceptions such as Balistes, not imbedded in sockets nor replaced vertically.

In some fish beak-like structures occur, formed partly of teeth, partly of the underlying jaw bones. These beaks are of two kinds: (1) In Scarus, the parrot fish, the premaxillae and dentaries bear numerous small, separately developed teeth, which are closely packed together and attached by their proximal ends to the bone, while their distal ends form a mosaic. Not only the teeth but the jaws which bear them are gradually worn away at the margins, while both grow continuously along their attached edge. (2) In Gymnodonts, e.g. Diodon, the beaks are formed by the coalescence of broad calcified horizontal plates, which when young are free and separated from one another by a considerable interval.

In some Teleosteans the differentiation of the teeth into biting teeth and crushing teeth is as complete as in Lepidosteus. Thus in the Wrasse (Labrus), the jaws bear conical slightly recurved teeth arranged in one or two rows, with some of the anterior ones much larger than the rest. The bones of the palate are toothless, while both upper and lower pharyngeal bones are paved with knob-like crushing teeth; such pharyngeal teeth occur also in the Carp but are attached only to the lower pharyngeal bone, the jaw bones proper being toothless.

In Dipnoi the arrangement of the teeth is very similar to that in Holocephali. The mandible bears a single pair of grinding teeth attached to the splenials, and a corresponding pair occur on the palato-pterygoids. In front of these there are a pair of small conical vomerine teeth loosely attached to the ethmoid cartilage. The palato-pterygoid teeth of Ceratodus are roughly semicircular in shape with a smooth convex inner border, and an outer border bearing a number of strongly marked ridges. The teeth of the extinct Dipteridae resemble those of Ceratodus but are more complicated.

ENDOSKELETON.

Spinal column[41].

The spinal column of fishes is divisible into only two regions, a caudal region in which the haemal arches or ribs meet one another ventrally, and a precaudal region in which they do not meet.

The various modifications of the spinal column in fishes can be best understood by comparing them with the arrangement in the simplest type known, namely Amphioxus. In Amphioxus the notochord is immediately surrounded by a structureless cuticular layer, the chordal sheath. Outside this is the skeletogenous layer, which in addition to surrounding the notochord and chordal sheath embraces the nerve cord dorsally, and laterally sends out septa forming the myomeres.

The Cartilaginous ganoids[42] Acipenser, Polyodon and Scaphirhynchus are the simplest fishes as regards their spinal column. The notochord remains permanently unconstricted and is enclosed in a chordal sheath, external to which is the skeletogenous layer. In this layer the development of cartilaginous elements has taken place. In connection with each neuromere, or segment as determined by the points of exit of the spinal nerves, there are developed two pairs of ventral cartilages, the ventral arches (basiventralia) and intercalary pieces (interventralia); and at least two pairs of dorsal pieces, the neural arches (basidorsalia) and intercalary pieces (interdorsalia). The lateral parts of the skeletogenous layer do not become converted into cartilage, so there are no traces of vertebral centra. The ventral or haemal arches meet one another ventrally and send out processes to protect the ventral vessels. The neural arches do not meet, but are united by a longitudinal elastic band.

In Cartilaginous ganoids the only indications of metameric segmentation are found in the neural and haemal arches. The case is somewhat similar with the Holocephali and Dipnoi.

In the Holocephali the notochord grows persistently throughout life, and is of uniform diameter throughout the whole body except in the cervical region and in the gradually tapering tail. The chordal sheath is very thick and includes a well-marked zone of calcification which separates an outer zone of hyaline cartilage from an inner zone. There are also a number of cartilaginous pieces derived from the skeletogenous layer which are arranged in two series, a dorsal series forming the neural arches and a ventral series forming the haemal arches. These do not, except in the cervical region, meet one another laterally round the notochord and form centra. To each neuromere there occur a pair of basidorsals, a pair of interdorsals, and one or two supradorsals. In the tail the arrangement is irregular.

In the Dipnoi as in the Holocephali the notochord grows persistently and uniformly, and the chordal sheath is thick and cartilaginous though there are no metamerically arranged centra. The neural and haemal arches and spines are cartilaginous and interbasalia (intercalary pieces) are present. The basidorsalia and basiventralia do not in Ceratodus meet round the notochord and enclose it except in the anterior part of the cervical and posterior part of the caudal region.

In Elasmobranchii the chordal sheath is weak and the skeletogenous layer strong. Biconcave cartilaginous vertebrae are developed, and as is the case in most fishes, constrict the notochord vertebrally.

Two distinct types of vertebral column can be distinguished in Elasmobranchs[43]:

1. In many extinct forms and in the living Notidanidae, Cestracion, and Squatina, the dorsal and ventral arches do not meet one another laterally round the centrum, and consequently readily come away from it.

2. In most living Elasmobranchs the arches meet laterally round the centrum.

The vertebrae are never ossified but endochondral calcification nearly always takes place, though it very rarely reaches the outer surface of the vertebrae. Elasmobranchs are sometimes subdivided into three groups according to the method in which this calcification takes place:

1. Cyclospondyli (Scymnus, Acanthias), in which the calcified matter is deposited as one ring in each vertebra.

2. Tectospondyli (Squatina, Raia, Trygon), in which there are several concentric rings of calcification.

3. Asterospondyli (Notidanidae, Scyllium, Cestracion), in which the calcified material instead of forming one simple ring, extends out in a more or less star-shaped manner.

In Heptanchus the length of the vertebral centra in the middle of the trunk is double that in the anterior and posterior portions, and as the length of the arches does not vary, the long centra carry more of them than do the short centra.

In many Rays the skull articulates with the vertebral column by distinct occipital condyles.

In Bony Ganoids the skeletogenous layer becomes calcified ectochondrally in such a way that the notochord is pinched in at intervals, and distinct vertebrae are produced. Ossification of the calcified cartilage rapidly follows. In Amia the vertebrae are biconcave, in Lepidosteus they are opisthocoelous, cup and ball joints being developed between the vertebrae in a manner unique among fishes. The notochord entirely disappears in the adult Lepidosteus, but at one stage in larval life it is expanded vertebrally and constricted intervertebrally in the manner usual in the higher vertebrata, but unknown elsewhere among fishes.

The tail of Amia is remarkable from the fact that as a rule to each neuromere, as determined by the exit of the spinal nerves, there are two centra, a posterior one which bears nothing, and an anterior one which bears the neural and haemal arches, these being throughout the vertebral column connected with the centra by cartilaginous discs.

In most Teleosteans but not in the Plectognathi the neural arches are continuous with the centra, which are nearly always deeply biconcave.

In some cases many of the anterior vertebrae are ankylosed together and to the skull. The vertebrae often articulate with one another by means of obliquely placed flattened surfaces, the zygapophyses. The centrum in early stages of development is partially cartilaginous, but the neural arches and spines in the trunk at any rate, pass directly from the membranous to the osseous condition.

Fins.

The most primitive fins are undoubtedly the unpaired ones, which probably originally arose as ridges or folds of skin along the mid-dorsal line of the body, and passed thence round the posterior end on to the ventral surface, partially corresponding in position and function to the keel of a ship.

In long 'fish' which pass through the water with an undulating motion such simple continuous fins may be the only ones found, as in Myxine. To support these median fins skeletal structures came to be developed; these show two very distinct forms, viz. cartilaginous endoskeletal pieces, the radiale, and horny exoskeletal fibres, the fin-rays. Mechanical reasons caused the fin to become concentrated at certain points and reduced at intervening regions. Thus a terminal caudal fin arose and became the chief organ of propulsion, and the dorsal and ventral fins became specialised to act as balancing organs.

In some of the earlier Elasmobranchs, the Pleuracanthidae, the endoskeletal cartilaginous radiale are directly continuous with outgrowths from the dorsal and ventral arches of the vertebrae, and form the main part of the fin. In later types of Elasmobranchs the horny exoskeletal fin-rays have comparatively greater prominence. In bony fish, as has been already stated, the horny fibres are replaced by bony rays of dermal origin, and at the same time complete reduction and disappearance of the cartilaginous radiale takes place.

The Caudal fin.

The caudal region of the spinal column in fishes is of special importance. It is distinctly marked off from the rest of the spinal column by the fact that the ventral or haemal arches meet one another and are commonly prolonged into spines, while in the trunk region they do not meet but commonly diverge from one another.

In some fish the terminal part of the caudal region of the spinal column retains the same direction as the rest of the spinal column. The blade of the caudal fin is then divided into two nearly equal portions, and is said to be diphycercal. This condition is generally regarded as the most primitive one; it occurs in the Ichthyotomi, Holocephali, all living Dipnoi, Polypterus and some extinct Crossopterygii, and a few Selachii and Teleostei. It occurs also in deep-sea fish belonging to almost every group, and under these conditions obviously cannot be regarded as primitive, but must be looked on as a feature induced by the peculiar conditions of life.

In the great majority of fish the terminal part of the caudal region of the spinal column is bent dorsalwards, and the part of the blade of the caudal fin which arises on the dorsal surface is much smaller than is that arising on the ventral surface. Such a fin is said to be heterocercal.

Strictly speaking all fish whose tails are not diphycercal have heterocercal tails, but the term is commonly applied to two-bladed tails in which the spinal column forms a definite axis running through the dorsal blade, while the ventral blade is enlarged and generally forms the functional part of the tail. Such heterocercal tails are found in nearly all Elasmobranchii, together with the living cartilaginous Ganoidei, and many extinct forms belonging to the same order; Lepidosteus, Amia, and the Dipteridae among Dipnoi, have tails which, though obviously heterocercal, are not two-bladed.

The vast majority of the Teleostei and some extinct Ganoidei have heterocercal tails of the modified type to which the term homocercal is applied. The hypural bones which support the lower half of the tail fin become much enlarged, and frequently unite to form a wedge-shaped bone which becomes ankylosed to the last ossified vertebral centrum. The fin-rays then become arranged in such a way as to produce a secondary appearance of symmetry. Some homocercal fish such as the Perch have the end of the notochord protected by a calcified or completely ossified sheath, the urostyle, to which several neural and haemal arches may be attached, and which becomes united with the centrum of the last vertebra; in others such as the Salmon the end of the notochord is protected only by laterally placed bony plates.

The Skull.

It is often impossible to draw a hard and fast line between the cranium and the vertebral column. This is the case for instance in Acipenser (fig. 18, 16) among Chondrostei, in Amia among Holostei, and in Ceratodus and Protopterus among Dipnoi. The occipital region of the skull in Amia is clearly formed of three cervical vertebrae whose centra have become absorbed into the cranium, while the neural arches and spines are still distinguishable.