238 PALAEONTOLOGY. 



forming on each side a series a little curved with the convexity 

 outwards, and the interspace between the two series is very 

 narrow. The maxillary teeth are much smaller than the pala- 

 tal ones, have a rounded or subquadrate crown, are four in 

 number, and of subequal dimensions. The premaxillary teeth, 

 three in number on each side, are more remote and distinct 

 from the maxillary teeth than in Placodus rostratus and P. 

 laticejos ; their crowns are more elongated and conical than in 

 P. laticeps ; the prehensile power of the prolonged premax- 

 illary part of the jaw being obviously greater in Placodus gigas 

 than in P. laticeps or P. rostratus. The size of the last tooth 

 in P. laticeps surpasses that of any of the teeth in the other 

 species. In proportion to the entire skull, it is the largest 

 grinding tooth in the animal kingdom, that in 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. laticeps. 



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 



