CRETACEOUS ERA. 



85 



the specific features of all higher animals have been again and 

 again changed since that period, these humble creatures have 

 preserved the characters they then possessed shall we say, 

 through a continuing uniformity in the conditions under which 

 they have lived, while all other animals have been exposed to 

 circumstances productive of change ? 



All the ordinary and more observable orders of the inha- 

 bitants of the sea, except the cetacea, have been found in the 

 cretaceous formation zoophytes, radiaria, mollusks, Crustacea 

 (in great variety of species), and fishes in smaller variety. 

 Down to this period, the placoid and ganoid fishes had, as far 

 as we have evidence, flourished alone now they decline, and 

 we begin to find in their place fishes of two orders of superior 

 organization, those which predominate in the present creation. 

 These are osseous in internal structure, with corneous scales. 

 The enaliosaurians disappear in this formation, while the land 

 reptiles, so numerous in the two preceding periods, become 

 much diminished in numbers. Of the latter, one of the most 

 remarkable was the Mosasaurus, which seems to have held an 



FIG. 63. 



Skull of Mosasaurus. 



intermediate place between the monitor and iguana, and to 

 have been about twenty-five feet long, with a tail calculated 

 to assist it powerfully in swimming. 



Fuci abounded in the cretaceous seas, and confervae are found 

 enclosed in flints. Of terrestrial vegetation, as of terrestrial 

 animals, the specimens in the European area are comparatively 



