41 8 ORDERS OF MAMMALIA. 



the Dugongs (Halicore), often spoken of collectively as " sea- 

 cows," and forming the family of the Manatida. 



Fig. 346. Sirenia. Dugong (Halicore). 



The most important, if not the only, fossil remains which 

 can be referred with certainty to the Sirenia, are those upon 

 which the genus Halitherium has been founded. The upper 

 incisors in this genus are tusk-like, the lower incisors small, 

 and the molars furnished with tubercular crowns. Halitherium 

 appears to be in some respects intermediate between the 

 Dugongs and Manatees ; and several species of the genus are 

 known, ranging from the Eocene to the Pliocene Tertiary. 



The genus Deinotherium referred to this order by De Blain- 

 ville, and still retained in this position by Pictet, will be here 

 considered as belonging to the order of the Proboscidca. 



ORDER V. CETACEA. In this order are the Whales, Dol- 

 phins, and Porpoises, all agreeing with the preceding in their 

 complete adaptation to an aquatic life. The body is com- 

 pletely fish-like in form ; the anterior limbs are converted into 

 swimming-paddles or " flippers ; " the proximal bones of the 

 fore-limbs are much reduced in length, and the succeeding 

 bones are shortened and flattened, and are enveloped in a 

 tendinous skin, thus reducing the limbs to oar-like fins ; there 

 are no external ears ; the posterior limbs are completely absent; 

 and there is a powerful, horizontally-flattened, caudal fin, some- 

 times accompanied by a dorsal fin as well. In all these char- 

 acters the Cetacea agree with the Sirenia, except in the one 

 last mentioned. On the other hand, the nostrils, which may 

 be single or double, are always placed at the top of the head, 

 constituting the so-called " blow-holes " or " spiracles ; " and 

 they are never situated at the end of a snout. The body is 

 very sparingly furnished with hairs, or the adult may be com- 

 pletely hairless. The teats are two in number and are placed 

 upon the groin. The head is generally of disproportionately 

 large size, and is never separated from the body by any distinct 

 constriction or neck. The lumbar region of the spine is long, 



