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. teeth in use. | cartilage. |
| 2. teeth in reserve. | 6. connective tissue. |
| 3. skin. | 7. mucous membrane of the |
| 4. cartilage of the jaw. | mouth. |
| 5. encrusting calcification of |
(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.
| 1. teeth in use. | a Sting-ray (Trygon) which has |
| 2. reserve teeth folded back. | pierced the jaw and affected the |
| 3. part of the caudal spine of | growth of the teeth. |
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