SAUROPTERYGIA. 229 



or the crocodiles less, in such and such characters. The truer 

 expression would, be that the lizards, which are the predomi- 

 nating form of Saurians at the present day, have retained more 

 of the osteological type of the triassic and oolitic reptiles, and 

 that the crocodiles have deviated further from them, or exhibit 

 a more modified or specialized structure. The backward posi- 

 tion of the external nostrils, the small size and position of the 

 palato-pterygoid foramina, are marks of affinity to Plesiosaurus, 

 in common with which genus the cranial structure of the 

 Ichthyosaurus exhibits a majority of lacertian characters. 



Order 4. — Sauropterygia.* 



No post-orbital and supra-temporal bones : large temporal 

 and other vacuities between certain cranial bones ; a 

 foramen parietale ; two antorbital nostrils ; teeth simple, 

 in distinct sockets of the prem axillary, maxillary, and 

 premandibular bones, rarely on the palatine or pterygoid 

 bones ; maxillaries larger than premaxillaries. Limbs 

 natatory ; not more than five digits. An episternum 

 and clavicles. A sacrum of one or two vertebrae for the 

 attachment of the pelvic arch in some, numerous 

 cervical vertebrae in most. Pleurapophyses with simple 

 heads ; those of the trunk long and bent. 



Genus Nothosaurus, Minister. 



Sp. Nothosaurus mirabilis, Minister. In fig. 90 is given 

 an analysis of the chief characters as yet ascertained of the 

 species which may be regarded as the type of its genus ; by 

 comparing this diagram with that of the Archegosaurus (fig. 

 84), the advance in the organization of the aquatic reptiles 

 will be readily traced and understood. 



The skull is no longer defended by a continuous covering 



* "Sxvpo;, a lizard ; vr\.fv% a fin. 



