230 The Chalk. 



strongest resemblance, consisting, among others, of 

 many species of Inoceramus, Lima, Pecten, Oyster, 

 Spondylus, Radiolites, Trigonia, &c. Being a deep- 

 sea deposit, it is poor in Gasteropoda, but rich in 

 Cephalopoda, especially in Nautili (AT. elegans, &c.), 

 Ammonites (A. Rothomagensis, &c.), and Turrilites 

 (T. costatus}, besides Baculites, Hamites simplex, 

 Scaphites (S. cequalis), and Belemnites. 



Numerically as individually, though still very 

 characteristic, Cephalopoda are less numerous in the 

 Cretaceous than in the Oolitic and Liassic strata, 

 though the latter contain fewer genera. In the Lias 

 and Oolites there are nearly 300 species of Cephalopoda, 

 most of which are Ammonites. In the Cretaceous 

 rocks less than 200 species are known, about 70 of 

 whith are Ammonites. More than 80 species of fish 

 are known in the Chalk, including all the four orders of 

 Agassiz, . Placoids, Granoids, Cycloids, and Ctenoids. 

 Many of the Placoids are Cestraciont fish, numerous 

 species being of the genus Ptychodus. Ten genera of 

 reptiles are known, two of which are allied to the 

 Crocodilia, Acanthophilis horridus, and Leiodon 

 anceps ; the great Mosasaurus, 3 species ; Plesiosaurus, 

 2 species, Ichthyosaurus and Pterodactyle, one of 

 which is said to have measured eighteen feet across the 

 expanded wings. Several Turtles occur in the Chalk, 

 Chelone Benstedi, &c. 



Having thus briefly described the Upper Cretaceous 

 strata of England, I shall next endeavour to show 

 what inferences may be drawn with regard to the 

 physical geography of the British area, during the time 

 occupied by their deposition. 



We have already seen that, during the deposition of 

 the Purbeck and Wealden strata, England formed part 



