328 Royal Instiiuiion. 



called " enamel." The characteristics of these, three primary tissues 

 of a tooth were briefly defined : they differ in hardness, the cement 

 being least dense, the enamel most. 



The tubular structure of the dentine relates to the disposition of 

 the hard material so as best to resist ])res£ure, and to the circulation 

 of plasma, transuded from the pulp throuj^h the dentine, so as to 

 maintain a certain, though languid, vitality of the tissue. 



Some secondary modifications of the chief tissue of teeth were 

 noticed under the names of osteo-dentine, vaso-dentiuc, vitro-dentine, 

 dendro-dentine, and labyrintho-dentine ; the latter highly complex 

 and beautiful modification being due rather to a modification of dis- 

 position, than of composition of the dentine itself. The singular laby- 

 ririthic interblending of the dentine and cement reaches its maximum 

 of complexity in the teeth of some gigantic extinct batrachian lleptiles, 

 from the Triassic formations, called from their distinct peculiarities, 

 " Labyrinthcdonts." 



The chief varieties in the form of the teeth in Fishes were then 

 enumerated, and more especially illustrated in the predatory Pike, 

 »he vegetarian Carp, the shell-crushing Myliobates, and the coral- 

 browsing Scarus. The elastic attachment of the teeth of the Lophius, 

 and the mode of growth and succession of the Shark's numerous teeth 

 were explained. 



From the class of Reptiles examples of dental structure were 

 selected from the Serpent-tribes, in relation to the poison-apparatus, 

 and from the Crocodile, in respect of the constant succession and dis- 

 placement of the teeth. The structure of the teeth of the extinct 

 Iguanodon and Megalosaurus was also noticed. 



The Mammalian class might be divided, in regard to the succession 

 of the teeth, into two groups — the Monuphyodonts, or those that gene- 

 rate but one set of teeth, and the Biphyodonts, or those that generate 

 two sets of teeth. 



The ^Monophyodonts include the Cetacea and the Bruta {Edentata 

 of Cuvier) ; all the other Orders are Uiphyodonts. 



The teeth of the Mammalia, especially the Diphyodonts, have 

 usually so much more definite and complex a form than those of 

 fishes and reptiles, that three parts are recognised in them ; viz. the 

 "fang," the "neck," and the "crown," The fang or root (radix) 

 is the inserted part ; the crown (corona) the exposed part ; and the 

 constriction which divides these is called the neck (cervix). I'he 

 term " fang" is properly given only to the implanted part of a tooth 

 of restricted growth, which fang gradually tapers to its extremity ; 

 those teeth Avhich grow uninterruptedly have not their exposed part 

 separated by a neck from their implanted j)art, and this generally 

 maintains to its extremity the same shape and size as the exposed 

 crown. 



It is peculiar to the class Mammalia to have teeth in)planted in 

 sockets by two or more fangs ; but this can only happen to teeth of 

 limited growth, and generally characterizes the molars and premolars : 

 jjerpetually growing teeth require the base to be kept simple and 

 widely excavated for the persistent pulp. In no mammiferous animal 



