OF THf 



UNIVERSITY 



THE ALLIES OS\THE SQUIRR^t: MAMMALIA 413 



Cetaceans are found in al seas, and feed on fishes, crus- 

 taceans, and the smaller floating animal life of the ocean gen- 

 erally. They vary from four to eighty feet in length, some 

 of the whales being the largest of existing animals. There is 

 every reason to believe that the group is descended from land- 

 mammals. The whalebone-whales are species without teeth, 

 but with a development of baleen or whalebone in the upper 

 jaw, which acts as a strainer. By means of the closely set, 

 flexible strips of whalebone the small animals on which they 

 feed are retained, while the water is forced out. Several 

 species are found in the Atlantic and Pacific oceans. The 

 sperm-whale (Physe'ter macroceph' alus) has a square head, 



FIG. 218. Narwhal. (After Cuvier) 



within which is a cavity containing oil, which on being refined 

 yields spermaceti. The narwhal (Mon'odon monoc'eros, Fig. 218) 

 is a smaller species, the males of which possess a single abnor- 

 mally developed incisor tooth, usually on the left side, which 

 grows into a tusk six or eight feet in length. 



Hoofed Mammals. The great assemblage of animals called 

 Ungula'ta (Lat. unguis, a nail) includes the hippopotami, 

 pigs, camels, deer, the giraffes, antelopes, oxen, goats, sheep, 

 rhinoceroses, horses, and elephants. All these mammals have 

 the toes ending in either a blunt nail or a fully developed 

 hoof, both of which structures are formed from the thicken- 

 ing of the skin of the toes. In ungulates like the cow and 

 sheep there are two divisions in the hoof, and the animals 

 really walk on the tip of the third and fourth digits, the others 

 being much reduced in size. In the horse and its allies this 

 reduction has gone much farther, so that the tip only of the 



