752 MAMMALIA. 



Order SIRENIA. Sea-Cows 



A small decadent order of sluggish, aquatic, vegetarian 

 Mammals, in no direct way connected with Cetaceans, to 

 which they have some superficial resemblance (convergence). 

 There are two living genera, Halicore (Dugong) and 

 Manatus (Manatee), and one was recently exterminated 

 (Rhytina). 



The Sirenia are sluggish, with massive heavy bones, a 

 plump body, some oil, and sparse hair on the thick tough 

 skin. In adaptation to aquatic life, they have a fish-like 

 form, a powerful tail with a " caudal fin," no external trace 

 of hind-limbs, flipper-like fore-limbs, no external ear, yalved 

 nostrils at the end of the snout, networks (retia mirabilia) 

 in the arteries (useful in prolonged immersion). They are 

 herbivorous, feeding on algae and estuarine plants; 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 (cf. Prototheria), and the small brain with few 

 convolutions. 



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

 nails ; the digits have never more than three phalanges, and 

 the elbow and wrist joints are distinctly movable, whereas 

 in the Cetacea the fore-limbs are more or less stiff from the 

 shoulder. There are no clavicles. 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 hind-limbs. The pelvis is rudi- 

 mentary, and there is no sacrum. In the extinct Hali- 

 therium there was a vestigial femur. 



The small eyes have imperfect eyelids, but have a nicti- 

 tating membrane. In the mouth there are horny crushing 

 plates. The ventricles are separated by a cleft. The 

 uterus is bicornuate. Two teats lie behind the armpits. 

 The placenta of the dugong is zonary, wholly or in great 

 part non-deciduate. The placenta of the manatee has not 

 yet been investigated, 



