56 ANIMAL LIFE PAST AND PRESENT. 



from the ancestral Lizards which have taken to. an 

 aquatic mode of life. 



The Mosasaurs are confined to the Cretaceous epoch, 

 and thus lived side by side with the Ichthyosaurs for 

 the greater part of their term of existence, although 

 they attained their maximum development in the very 

 highest Cretaceous beds when the Ichthyosaurs and 

 Plesiosaurs seem to have disappeared. For a short 

 time, then, these creatures appear to have been the 

 only gigantic marine Vertebrates, filling up the gap left 

 by the disappearance of the Ichthyosaurs, which had 

 not yet been occupied by the Whales. They were of 

 carnivorous habits, as shown by their formidable teeth, 

 and, like all groups of Vertebrates which have taken 

 to a marine life, far exceeded in dimensions any of 

 their terrestrial cousins, the length of some of the 

 fepecies being as much as forty feet. 

 \J We come now to the Birds, several groups of which 

 are exclusively composed of species specially adapted 

 for an aquatic life. Swimming birds, as a rule, support 

 themselves on the surface of the water, taking occa- 

 sional dives of longer or shorter duration, and, there- 

 fore, have no need to make any especial arrangements 

 for breathing. They swim by using their legs as oars, 

 the feet being webbed, and the toes folding up as the 

 foot is brought forward after one stroke to prepare for 

 a second. As we shall see, however, some species aid 

 their swimming with their wings. All birds that 

 swim have relatively short legs, which are generally 

 placed far back on the body, since this position gives 



