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, — Halcore (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, valved 
nostrils at the end of the snout, networks (retia mirabilia) 
in the arteries (useful in prolonged immersion). They are 
herbivorous, feeding on alge 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 vertebree (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 Au/i- 
thertum 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. 
