Sometimes it is made quinquelobate by a double notch on each side of the large middle lobe (Boops). In the formidable Sea-pike (Sphyrcena Barracuda) the crown of each tooth, large and small, is produced into a compressed and sharp point, and resembles a lancet. Sometimes the edges of such lancet shaped teeth are finely serrated, as in Priodon, and the great Sharks of the genus Carcharias, the fossil teeth of which indicate a species (Garth. Illegalodon) sixty or seventy feet in length.
The lancetted form is exchanged for the stronger spear-shaped tooth in the Sharks of the genus Lamna ; and in the allied great ex tinct Otodus, as in the small Porbeagle, simi larly shaped, but stronger, piercing and cut ting teeth were complicated by one or more accessory compressed cusps on each side their base, like the Malay crease.
With respect to situation, the teeth, in Sharks and Rays, are limited to the bones (maxillary and mandibular), which form the anterior aperture of the mouth : in the Carp and other Cyprinoids the teeth are confined to the bones (pharyngeal and basi-occipital) which circumscribe the posterior aperture of the mouth. The Wrasses (Labrus) and the (Scares) have teeth on the pre-maxillary and pre-mandibular, as well as on the upper and lower pharyngeals ; both the anterior and posterior apertures of the mouth being thus provided with instruments for seizing, dividing, or comminuting the food, the grinders being situated at the pha rynx. In most fishes teeth are developed also in the intermediate parts of the oral cavity, as on the palatines, the vower, the hyoid bones, the branchial arches ; and, though less commonly, on the pterygoids, the entopterygoids, the sphenoids, and even on the nasal hone (fig. 560, c.). It is very rare to find teeth developed on the true superior maxillary bones ; but the Herring and Salmon tribes, some of the Ganoid Fishes, and the great SudiJ, are examples of this approach to the higher Vertebrata. Among the anoma lous positions of teeth may be cited, besides the occipital alveolus of the Carp*, the mar ginal alveoli of the prolonged, depressed, well ossified rostrum of the Saw-fish, Pristis. In the Lampreys and in Helostomus (an os seous fish), most of the teeth are attached to the lips. Lastly, it is peculiar to the class Pisces, amongst Vertebrata, to offer examples of teeth developed in the median line of the mouth, as in the palate of the Myxines (fig. 559, a.); or crossing the symphysis of the jaw, as in Notidamis, Seyninus, and Myliobates.
Nor is the mode less varied than the place of attachment. The teeth of Lophius, Par cilia, Anableps, are always moveable. In most fishes they are anchylosed to the jaws by continuous ossification from the base of the dental pulp ; the histological transition being more or less gradual from the structure of the tooth to that of the bone. Sometimes we find, not the base, but one side of the tooth anchylosed to the alveolar border of the jaw ; and the teeth oppose each other by their sides instead of their summits (Scants*); in Pintelodus, however, where the teeth are thus attached, the crown is bent down in the upper teeth, and bent up in the lower ones, at right angles to the fang, so that they oppose each other by the normal surfaces. Certain teeth of recent and fossil cartilagi nous fishes have their base divided into processes like fangs, but these serve for the attachment of ligaments, and are not set in bony sockets like the true fangs or roots of the teeth of the Mammalia. Some Sharks have two divaricating fangs ; some fossil teeth referred to my genus Petalodus by Agassiz, with the specific name "radicans," have the base divided into several fangs or processes, indicating a generic distinction. The base of anchylosed teeth is, at first, attached to the jaw-bone by ligament ; and in the Cod fish, Wolf-fish, and some other species, as calcification of the tooth progresses towards its base, the subjacent portion of the jaw bone receives a stimulus, and developes a process corresponding in size and form with the base of the tooth : for some time a thin layer of ligamentous substance intervenes, but anchylosis usually takes place to a greater or less extent before the tooth is shed. Most of
the teeth of the Lophius retain the primitive ligamentous connection ; the ligaments-- of the large internal or posterior teeth of the upper and lower jaws, radiate on the corre sponding sides of the bone, the base of the tooth resting on a conformable alveolar pro cess. The ligaments do not permit the tooth to be bent outwards beyond the vertical posi tion, but yield to pressure in the contrary direc tion, by which the point of the tooth may be directed towards the back of the mouth ; the instant, however, that the pressure is remitted, the tooth returns through the elasticity of the bent ligaments, as by the action of a spring, into its usual erect positions (fig. 512, c. c. Vol. III. p. 978. art. PiscEs) ; the deglutition of the prey of this voracious fish is thus facili tated, and its escape prevented.t The broad and generally bifurcate bony base of the teeth of Sharks is attached by ligament to the semi ossified crust of the cartilaginous jaws §; hut they have no power of erecting or de pressing the teeth at will. The small and closely crowded teeth of Rays are also con nected by ligaments to the subjacent maxillary and mandibular membranes. The broad tes selated teeth of the have their attached surface longitudinally grooved to afford them better hold-fast, and the sides of the contiguous teeth are articulated together by serrated or finely undulating sutures, a structure unique in dental organisation. II The teeth of the Sphyrcena are examples of the ordinary implantation in sockets, with the addition of a slight anchylosis of the base of the fully-formed tooth with the alveolar pa rietes ; and the compressed rostral teeth of the Saw-fish are deeply implanted in sockets. In the latter the hind margin of their base is grooved, and a corresponding ridge from the back part of the socket fits into the groove, and gives additional fixation to the tooth. Some implanted teeth in the present class have their hollow base further supported, like the claws of the feline tribe, upon a bony process aris ing from the base of the socket ; the incisors of the Balistes, e.g., afford an example of this double or reciprocal gomphosis. In fact, the whole of this part of the organisation of fishes is replete with beautiful instances of design, and instructive illustrations of animal me chanics. The vertical section of a pharyngeal jaw and teeth of the Wrasse (Labrus *) would afford the architect a model of a dome of unusual strength, and so supported as to re lieve from pressure the floor of a vaulted chamber beneath. The base of the dome shaped tooth is slightly contracted, and is implanted in a shallow circular cavity ; the rounded margin of which is adapted to a cir cular groove in the contracted part of the base; the margin of the tooth which imme diately transmits the pressure of the bone, is strengthened by an inwardly projecting con vex ridge. The masonry of this inner but tress, and of the dome itself, is composed of hollow columns, every one of which is placed so as best to resist or transmit in the due direction the external pressure. The floor of the alveolus is thus relieved from the office of sustaining the tooth : it forms, in fact, the roof of a lower vault, in which the germ of a successional tooth is in course of develop ment ; had the crushing tooth in use, rested, as in the Wolf-fish, by the whole of its base upon the alveolus, the supporting plate, gra dually undermined by the growth of the new tooth, must have given way, and been forced upon the subjacent delicate and highly vas cular and sensitive matrix of the half-formed tooth. But the superincumbent pressure is exclusively sustained by the border of the alveolus, whence it is transferred to the walls dividing the vaulted cavities containing the germs of the new teeth ; the roofs of these cavities yield to the absorbent process consequent on the growth of the new teeth without materially weakening the attachment of the old teeth, and without the new teeth being subjected to any pressure until their growth is sufficiently advanced to enable them to bear it with safety ; by this time the sus taining borders of the old alveolus are under mined, and the old worn-down tooth is shed.