INTRODUCTION. Ixix 



by peculiar forms of teeth ; but a definite dental formula can rarely 

 be assigned as a generic character of a Reptile. There is no de- 

 cided modification of dental structure peculiar to any of the class 

 of Reptiles ; the poison-fang is rather a modification of form. 

 The labyrinthic structure reaches its maximum of complexity in 

 the great extinct Sauroid Batrachians of the Keuper Sandstones, 

 but ** it also exists at the base of the tooth in a few Fishes/* 

 (Part II, p. 201), and specific instances of it in that class {Lepi- 

 dosteus, and a few other Sauroids) have, since Part II. was published, 

 received illustrations in the works of Prof. Agassiz(l) and Dr. 

 'Wyman.(2) The only constant and general character of the teeth of 

 the cold-blooded classes of Vertebrata is derivable from the brief 

 period of their existence in the individual, so that the few teeth which 

 develope roots have these always simple and undivided, usually 

 hollow, and with the germ of a successor in or near them. 



With the exception of the composite dental masses of the 

 Chimseroids, and the anomalous rostral teeth in Pristis, no existing 

 species of Fish or Reptile could be said to have permanent teeth ; 

 and no extinct species of either class has yet been found with teeth 

 having divided roots implanted in sockets, or manifesting evidence 

 of perpetual growth by a persistent pulp, excepting the singular 

 extinct Saurians of South Africa, with two long canine tusks in the 

 upper jaw, which must have grown and been maintained throughout 

 life, of due size and strength, like the tusks of the Boar and 

 Walrus. (3) With the exception of these two anomalous teeth, 

 the jaws of the Dicynodonts were edentulous. 



In the Mammalian Class the value of the dental organs, as 



(1) Poissons Fossiles, Notice surles Sauroides, Janvier, 1843. 



(2) Trans. Boston Society of Natural History, August, 1843. 



(3) Memoir on the Dicynodon, Geological Transactions, 2nd series, vol. vii. 



