218 PALEONTOLOGY 



ously discovered species. In proportion to the entire skull, 

 it is the largest grinding tooth in the animal kingdom, the 

 elephant itself not excepted. 



All these teeth are implanted by short simple bases in 

 distinct hollow sockets, subject to the same law of displace- 

 ment and succession as in other reptiles. By some it may be 

 deemed requisite to separate generically the Placodi with two 

 teeth from those with three teeth in each palatal series ; but 

 the Placodus rostratus offers a transitional condition in the 

 small relative size of the first two palatal teeth, and in the 

 rounded form of all the teeth, from the P. Andriani to the 

 P. laticc]js. 



We cannot contemplate the extreme and peculiar modifi- 

 cation of form of the teeth in the genus Placodus without a 

 recognition of their adaptation to the pounding and crushing 

 of hard substances, and a suspicion that the association of the 

 fossils with shell-clad Mollusks in such multitudes as to have 

 suggested special denominations to the strata containing 

 Placodus [e.g., muschelkalk, terebratulitenkalk, etc.), is indi- 

 cative of the class whence the Placodi derived their chief 

 subsistence. 



No doubt the most numerous examples of similarly-shaped 

 teeth for a like purpose are afforded by the class of fishes, as, 

 e.g., by the extinct Pycnodonts, and by the wolf-fish (Anar- 

 rhichas lupus) and the Cestracion of the existing seas. But 

 the reptilian class is not without its instances at the present 

 day of teeth shaped like paving-stones, of which certain 

 Australian lizards exhibit this peculiarity in so marked a 

 degree that the generic name Cydodus has been invented to 

 express that peculiarity. Amongst extinct reptiles, also, a 

 species of lizard from the tertiary deposits of the Iimagne 

 in France presents round obtuse teeth, of which the last, in 

 the lower jaw, is suddenly and considerably large] than the 



Vest. 



