SAUROPTERYGIA 219 



Nothosaurus, Simosaurus, and Pistosaurus present the same 

 evidences of lacertian affinities in the division of the nostrils 

 by the median extension of the prernaxillary backwards to 

 the nasals, the same thecodont dentition, and the same circum- 

 scription of the orbits and temporal fossas as in Placodus : 

 there is also a general family likeness hi the upward aspect 

 of these apertures, accompanying an extreme depression of the 

 skull. The muzzle, though varying greatly in length in these 

 genera, presents the same obtuseness ; and the alveolar border 

 of the jaws the same smooth outward convexity which we 

 observe in the Placodus. The peculiar confluence of the 

 elements of the upper and lower zygomatic arches, — i.e., of 

 post-frontal and malar, — forming the broad wall of bone behind 

 the orbit, is continued still farther backwards in the Simosaurus. 

 In Pistosaurus the elongated post-frontal, malar, and squa- 

 mosal are united together in one deep zygomatic arch, 

 which has the mastoid and tympanic for its hinder abut- 

 ment. 



It is remarkable that hitherto no vertebras or other bones 

 of the trunk or limbs have been found so associated with the 

 teeth of Placodus, as to have suggested their belonging to the 

 same species. Usually, after the indication of a reptile by 

 detached teeth, the next step in its reconstruction is based 

 upon detached vertebras. The twelve or more evidences of 

 Placodus, afforded by bone as well as tooth, are all portions of 

 the skull. It is possible that some of the singularly modified 

 vertebras from the muschelkalk, next to be described, may 

 belong to the Placodus ; and the same surmise suggests itself 

 in reference to some of the limb-bones from the muschelkalk 

 that cannot be assigned to other known saurian genera. 



The obvious adaptation of the dentition of Placodus to the 

 crushing of very hard kinds of food, its close analogy to the. 

 dentition of certain fishes known to subsist by breaking the 

 shells of whelks and other shell-clad Mollusks, and the cha- 



