252 THE OSTEOLOGY OF THE REPTILES 



As might be suspected in such forms, the number of presacral verte- 

 brae is reduced. 



The temporal vacuity is bounded as in the plesiosaurs, and also in 

 some theriodonts. The maxillae are large, the nares situated rather 

 far back, perhaps an adaptation for grubbing in the mud after in- 

 vertebrates. Possibly there was a moderate adaptation in Placodus 

 for life in shallow water. 



The placodonts were reptiles of considerable size, perhaps eight or 

 ten feet in length, undoubtedly slow in movement, and with a heavy 

 skull, as have all shell-eating reptiles. 



Until more is known of the skeleton, the group may remain in an 

 independent position, though there is Uttle in the structure of the 

 skull that would entitle them to an ordinal rank; shell-eating animals 

 with crushing teeth occur in various orders. 



Family Placodontidae. Upper Triassic. Placodus Agassiz {Ano- 

 mosaurus Huene), Placochelys Jaekel, Cyamodus Meyer, Europe. 



