CRETACEOUS FORMATIONS. 25 



to the prevailing type of the sea-lizards of the great geological epoch then draw- 

 ing to its close. 



The seas in which the English Chalk hills and cliffs were formed, and by which 

 they were modified in the course of upheaval, must have teemed with life, and 

 have been traversed by shoals of fishes needed for the sustentation of the numer- 

 ous kinds of large marine reptiles now known to have existed during that period, 

 and all of which were provided with jaws and teeth adapted, under diverse secon- 

 dary modifications, to the capture and destruction of the finny races. Of these 

 carnivorous reptiles some, as, e. g., Ichthyosaurus campylodon and Plesiosaurus 

 Bemardi, were large species of genera represented throughout the oolitic period ; 

 others, as, e. g., Leiodon and Mosasaurus, offer generic or family modifications 

 of the Saurian structure, unknown in any other than the Cretaceous deposits. 

 The subject of the present section, as gigantic as the Maestricht Mosasaur, mani- 

 fests an extreme modification of the Plesiosauroid type of structure. It is pro- 

 bable that the large Pterodactyles of the same geological period, soaring like 

 albatrosses and giant petrels over the Cretaceous seas, co-operated with the marine 

 reptiles, as those sea birds now do with cetaceous mammals, in reducing the ex- 

 cessive numbers of the teeming tribes of fishes, and in maintaining the balance of 

 oceanic life. 



