694 



MAMMALIA. 



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 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. In the extinct Halitherium 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. 



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

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

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

 been exterminated in the last century. 



The order was once much larger. Fossil forms occur in Tertiary 

 strata. The most important is Halitherium, a less specialised Sirenian 

 than those still extant, e.g. with traces of hind-limbs. 



