THE CRETACEOUS PERIOD. 



forms. The recent order of the Chelonians is also represented 



in the Cretaceous rocks, 

 by forms closely re- 

 sembling living types. 

 Thus the fresh-water 

 deposits of the Wealden 

 have yielded examples 

 of the " Terrapins " or 

 "Mud-Turtles" (Emys) ; 

 and the marine Creta- 

 ceous strata have been 

 found to contain the 

 remains of various spe- 

 cies of Turtles, one of 

 which is here figured 

 (fig. 211 ). No true 

 Serpents (Ophidia) have 

 as yet been detected in 

 the Cretaceous -rocks; 

 and this order does not 

 appear to have come 

 into existence till the 

 Tertiary period. Last- 

 ly, true Crocodiles are 

 known to have existed 

 in considerable num- 

 bers in the Cretaceous period. The oldest of these occur 

 in the fresh-water deposits of the Wealden ; and they differ from 

 the existing forms of the group in the fact that the bodies 

 of the vertebrae, like those of the Jurassic Crocodiles, are 

 bi-concave, or hollowed out at both ends. In the Greensand 

 of North America, however, occur the remains of Crocodiles 

 which agree with all the living species in having the bodies of 

 the vertebrae in the region of the back hollowed out in front 

 and convex behind. 



Birds have not hitherto been shown, with certainty, to have 

 existed in Europe during the Cretaceous period, except in a 

 few instances in which fragmentary remains belonging to this 

 class have been discovered. The Cretaceous deposits of 

 North America have, however, been shown by Professor 

 Marsh to contain a considerable number of the remains of 

 Birds, often in a state of excellent preservation. Some of 

 these belong to swimming or Wading Birds, differing in no 

 19 



Fig. 211. Carapace of Chelone Benatedi. 

 Lower Chalk . ( After Owen . ) 



