90 GEOLOGY. 



The suborder of crocodilians, to which the name Thalattosuchia 

 has recently been applied by Fraas, made its appearance during the 

 latter part of the period, but enjoyed only a brief existence. These 

 truly marine crocodiles had undergone a remarkable adaptation to 

 sea life, from the land or fresh- water forms (Fig. 368). They were very 



Fig. 368. — Restoration of a Jurassic crocodilian, Geosaurus suevicus. (Fraas.) 



fish-like in appearance, were wholly covered with a bare skin, and the 

 long tail terminated in a large fin, like that of the ichthyosaurs. The 

 eyes were protected by sclerotic plates, and the fore limbs were short 

 and quite paddle-like. The hind limbs, however, were only slightly 

 modified from the land type, perhaps due to the recurring necessity 

 of visiting the shores for depositing and hatching their eggs. 



True marine turtles, so characteristic of the Cretaceous, had not 

 yet appeared, though before the close of the period a number of forms 

 had arisen presenting a strange admixture of characters peculiar to 

 fresh- water and sea turtles (Thalassemydce). 



The American marine faunas. — The marine life of Jurassic times is but feebly 

 represented in the American strata, no representatives at all having been found 

 on the eastern coast. There was doubtless a sea-shelf on that border which 

 was occupied by its appropriate fauna, but it has been buried by later deposits. 



In the Pacific region, marine life occupied nearly the same districts as in 

 Triassic times, but no consecutive series of faunal evolution has yet been worked 

 out. Present imperfect evidence points to two faunal provinces, one of which 

 succeeded the southern or Nevada-California province of the Trias, and the 

 other the north Pacific province. The fauna of the former ranges from the 

 lower to the upper division, that of the latter represents the later Jura only. 

 The fauna of the earliest epoch (Lias) does not appear to have been derived from 



