CHAPTER XXYIII. 
Manatis and Dugongs,— Order Sirenia. 
The purely aquatic mammals known as manatis and dugongs, together with the 
northern sea-cow, which has become extinct within the last century and a half, 
constitute an order by themselves, and may be collectively known as Sirenians. 
Although they are as well fitted for an aquatic life as the Cetaceans, these animals 
have no sort of relationship with the members of that order, and have evidently 
been derived quite independently from terrestrial mammals. Such resemblances 
as do exist between Sirenians and Cetaceans are entirely of an adaptive nature, and 
have been produced merely by the two groups of animals leading a somewhat 
similar mode of life. 
Although the existing Sirenians resemble the Cetaceans in having 
Characteristics. . . ® 
their fore-limbs converted into nippers, and having lost all traces of 
the hind-limbs, while the tail is converted into a horizontally-expanded rudder¬ 
like organ, comparable to the flukes of the whales and dolphins, their general 
conformation is very different. In the first place, although the body is somewhat 
cetacean-like, without any well-defined neck and with no distinction between 
trunk and tail, it is markedly depressed, instead of being more or less com¬ 
pressed from side to side. Then, again, the head departs but little from the 
ordinary mammalian type, being comparatively small in proportion to the body, 
with the summit rounded, and the nostrils, which are double and capable of 
being closed at will by valve-like flaps, placed above the extremity of the 
abruptly-truncated muzzle. The back-fin, so commonly present in the Cetaceans, 
is totally wanting. In the flippers, although the whole of the toes are enclosed 
in a paddle-shaped mass of integument, traces of nails are still in some cases 
retained. The eyes are small, with imperfectly-developed lids, and the minute 
aperture of the ear is unprovided with any external conch. The mouth is small, 
with thick, fleshy lips, upon which grow a number of bristly hairs, which 
persist throughout life. The skin is thick, and either finely wrinkled or rugged 
and bark-like, sometimes with fine hairs thinly distributed upon it. The female 
has a single pair of teats placed on the breast. The teeth are very variable, being 
totally wanting in the northern sea-cow, while in the other two living genera they 
consist of incisors and cheek-teeth. The structure of the cheek-teeth is, however, 
very different in the two latter, and in one of them their number is much greater 
than among less aberrant mammals. The living forms have been recently discovered 
to possess rudimental milk-teeth, and in some extinct species such teeth were well 
developed. Certain extinct members of the order were, moreover, furnished with 
a complete set of teeth, comparable to those of ordinary mammals. All the recent 
