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. [Illustration FIG. 14. DIAGRAM OF A SECTION THROUGH THE JAW OF A SHARK (_Odontaspis americanus_) showing the succession of teeth (Brit. Mus. from specimen and diagram). 1. teeth in use. 2. teeth in reserve. 3. skin. 4. cartilage of the jaw. 5. encrusting calcification of cartilage. 6. connective tissue. 7. mucous membrane of the mouth.] (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.