TEETH OF FISHES. 375 



prevented. The broad and generally bifurcate bony base of the 

 teeth of Sharks is attached by ligament to the semiossified crust 

 of the cartilaginous jaws, fig. 263 ; but they have no power of 

 erecting or depressing the teeth at will. The small and closely 

 crowded teeth of Rays are also connected by ligaments to the 

 subjacent maxillary and mandibular membranes. The broad tes- 

 selated teeth of the Myliobates have their attached surface longi- 

 tudinally 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. 

 The teeth of the Sphyr&na are examples of the ordinary im- 

 plantation in sockets, with the addition of a slight anchylosis of the 

 base of the fully-formed tooth with the alveolar parietes ; and the 

 compressed rostral teeth of the Saw-fish, fig. 65, 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 arising from the base of the socket ; the in- 

 cisors of the Balistes, e.g. afford an example of this double or 

 reciprocal gomphosis. 1 In fact, the whole of this part of the 

 organisation of fishes is replete with beautiful instances of design 

 and instructive illustrations of animal mechanics. 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 relieve from pressure the floor of a vaulted 

 chamber beneath. The base of the dome-shaped tooth, fig. 240, 

 p 9 is slightly contracted, and is implanted in a shallow circular 

 cavity ; the rounded margin of which is adapted to a circular 

 groove in the contracted part of the base ; the margin of the 

 tooth which immediately transmits the pressure of the bone, is 

 strengthened by an inwardly projecting convex ridge. The 

 masonry of this inner buttress, 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 sus- 

 taining the tooth : it forms, in fact, the roof of a lower vault, in 

 which the germ of a successional tooth, fig. 261, b, is in course of 

 developement. The superincumbent pressure is exclusively 

 sustained by the border of the alveolus, whence it is transferred to 



1 V. p. 82, pi. 40. 



