HIGH SCHOOL ZOOLOGY. 181 



Tlie Phoridm chiefly differ from the above in the absence of external 

 ears and the shape of the flippers, those of the hinder extremities being 

 especially remarkable on account of their notched outline, due to the 

 shortness of the middle and the length of the inner and outer toes. 

 The harp seal of the St. Lawrence is one of the commonest species 

 {Phoca groenlandica, Fig. 195), but there are some singular forms also in 

 this family, such as the hooded seal of the North Atlantic (Cystophora 

 cristata), and the gigantic sea-elephant of the Antarctic Ocean, 

 (C. proboscidea), which attains a length of twenty feet, and is distin- 

 guished by the prolongation of the nose into a proboscis. Some species 

 of the family live in land-locked seas, such as the Caspian and the 

 lakes of Newfoundland. 



The Walrus ( Trichechus rosmarns) is placed in a family by itself, char- 

 acterized by the enormous upper canine tusks, by the shape of the 

 trunk, which does not diminish in girth towards the loins, by approach- 

 ing the Phocidte, in respect to the absence of external ears, and the 

 Otarise, in the ability of the creatures to raise the body on the limbs, 

 and thus leave the sea, and even climb steep rocks so as to reach a 

 safe place above high water, where they may bask in the sun. 



It is observed that the young sea-lions take somewhat unwill- 

 ingly to water, and swim at first awkwardly ; this is one indication, 

 among many others, that the Pinnipedia are a group of mammals 

 which have gradually acquired aquatic habits, and with them their 

 modified form. The same is thought to be true of the Sirenia, for fossil 

 forms have been found allying them to the terrestrial Ungulata. Great 

 uncertainty is felt by zoologists, however, as to the alliances of the 

 third group of aquatic mammalia — the Cetacea ; many, nevertheless, 

 think that these carnivorous aquatic forms may have been originally 

 more seal-like in form, and that the horizontal caudal fin had within it 

 the hard parts of the hinder extremities, of which hardly a trace is now 

 to be detected ; no fossils have yet been found to confirm this idea, 

 although various anatomical considerations render it probable. 



35. In turning to the other orders of Unguiculate mammals, 

 we shall find some forms that exhibit more primitive features 

 than the Carnivores, others that are specialised for an aerial, 

 arboreal or subterranean life, as much as the Pinnipedia are for 

 a life in water. 



