SEA COWS. 691 



Order 2. SIRENIA Sea Cows. 



A small moribund order of sluggish, aquatic, vegetarian 

 Mammals, in no direct way connected with Cetaceans, 

 possibly related to Ungulates, but certainly primitive. There 

 are two living genera, Halicore (Dugong), and Manatus 

 (Manatee), and one recently exterminated (Rhytina). 



The Sirenia are sluggish animals, with massive heavy bones, 

 a plump body, some oil, and sparse hair. They are aquatic, 

 with fish-like form, no trace of hind limbs, flipper-like fore 

 limbs, no external ear, valved nostrils, networks in the 

 arteries (useful in prolonged immersion). Th,ey are herbi- 

 vorous, and like others of similar habit have a chambered 

 stomach, a long intestine, and a caecum. 



They are primitive, and with this fact may be associated 

 the abdominal testes, the absence of distinct epiphyses on 

 the vertebrae (as in Prototheria), and the small, rather 

 smooth brain. 



The body is fish-like, the head rounded, the skin is 

 thick and tough, with sparse bristles, mostly about the 

 mouth. 



The paddle-shaped fore limbs have, at most, rudimentary 

 nails ; there are no hind limbs. The skull is not like that 

 of Cetaceans. The nasals are, at most, rudimentary. There 

 are no canine teeth. There are chevron bones below the 

 tail. There are no clavicles. The pelvis is rudimentary, 

 and there is no sacrum. 



The brain is small and has few convolutions. The nostrils 

 are valved, and lie at the end of the snout. The small 

 eyes have imperfect eyelids, but have a nictitating mem- 

 brane. There are no external ears. In the mouth there 

 are horny crushing plates. The stomach is chambered, 

 and there is a caecum. The ventricles are separated by a 

 cleft. There are retia mirabilia on the arteries. The testes 

 are abdominal in position. The uterus is bicornuate. Two 

 teats lie behind the arm pits. The placenta of the dugong 

 is zonary, wholly or in great part non-deciduate. The 

 placenta of the manatee has not yet been investigated. 

 The food consists of Algae and estuarine plants. 



