846 



PHYLUM CHORDATA : CLASS MAMMALIA 



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, rudimen- 

 tary 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 rudimentary, and there is no sacrum. In the 

 extinct Halitherium there was a vestigial femur. 



The small eyes have imperfect eyelids, but have a 

 nictitating 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 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 genus Rhytina was toothless, with a slightly crooked snout, 

 small head and arms, and thick naked skin. Steller's sea-cow {R. 

 stelleri) — the only known species, from the North Pacific — seems to 

 have been exterminated about 1768. The Tertiary Halitherium had 

 traces of hind-limbs. 



