DENTAL SYSTEM OF FISHES. 223 



tooth to be bent outwai'ds beyond the vertical position, but yield to 

 pressure in the contrary direction, by which the point of the tooth 

 may be directed towards the back of the mouth : the instant, how- 

 ever, 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 position : the deglutition of the px'ey of this voracious fish 

 is thus facilitated, and its escape prevented. 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 ; 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 tesselated 

 teeth of the Myliobates have their attached surface longitudinally 

 grooved to afford them better hold-fast, and the sides of the contiguous 

 teeth are articulated together by sei'rated or finely undulating sutures 

 (Y,pl. 27.), a structure unique in dental oi'ganisation. The teeth of 

 the Sphyrasna 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 parietes ; and the compressed rostral teeth of the 

 Saw-fish are deeply implanted in sockets : the hind margin of their 

 base is grooved, and a con-esponding 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 incisors of the Ba- 

 listes, e. g., aftbrd 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 mechanics. The vertical section of a pharyngeal jaw and 

 teeth of tlie 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 domeshaped 

 tooth 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 mai-gin 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 di- 

 rection the external pressure. The floor of the alveolus is thus re- 

 lieved from the oflice of sustaining the tooth : it forms, in fact, tlie 

 roof of a lower vault, in which the germ of a successional tooth is in 

 course of development : had the crushing tooth in use, rested, as in 

 the Wolf-fish, by the whole of its base upon the alveolus, the sup- 



