570 MANUAL OF ZOOLOGY. 



attenuated, the snout truncated and callous, and the ears large, 

 erect, and pointed. 



CHAPTER LXXVI. 



SIRENIA AND CETACEA. 



ORDER IV. SIRENIA. This order comprises no other living 

 animals except the Dugongs and Manatees, which are often 

 placed with the true Cetaceans (Whales and Dolphins) in a 

 common order. There is no doubt, in fact, but that the Sirerzia 

 are very closely allied to the Cetacea ; and though they are to 

 be regarded as separate orders, yet they may be advantage- 

 ously considered as belonging to a single section, which has 

 been called Miitilata, from the constant absence of the hind- 

 limbs. 



The Sirenia agree with the Whales and Dolphins in their 

 complete adaptation to an aquatic mode of life (fig. 243) ; 

 especially in the presence of a powerful caudal fin, which differs 

 from that of Fishes in being placed horizontally, and in being 

 a mere expansion of the integuments, not supported by bony 

 rays. The hind-limbs are wholly wanting, and there is no 

 sacrum. The anterior limbs are converted into swimming- 

 paddles or "flippers." The snout is fleshy and well developed, 

 and the nostrils are placed on its upper surface, and not on the 

 top of the head, as in the Whales. Fleshy lips are present, and 

 the upper one usually carries a moustache. The skin is covered 

 with fleshy bristles. The head is not disproportionately large, 

 as in the true whales, and is not so gradually prolonged into 

 the body as it is in the latter. There may be only six cervical 

 vertebrae. The teats are two in number and are "thoracic," 

 i.e., are placed on the chest. There are no clavicles, and 

 the digits have no more than three phalanges each. The testes 

 are retained throughout life within the abdomen, but vesiculse 

 seminales are present. The animal is diphyodont, the perma- 

 nent teeth consisting of molars with flattened crowns adapted 

 for bruising vegetable food, and incisors which are present in 

 the young animal, at any rate. In the extinct Rhytina it does 

 not appear that there were any incisor teeth. 



The only existing Sirenia are the Manatees (Manatus] and 

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

 cows," and forming the family of the Manatidce. 



