2145 SIRENTA 
owing to the very oblique position of the diaphragm, the thoracic 
cavity extends far back over the abdomen. The epiglottis and 
arytenoid cartilages of the larynx do not form a tubular prolong- 
ation as in the Cetacea, so that the epiglottis is not intranarial. 
The brain is of comparatively small size, and the convolutions on 
the surface of the cerebrum are few and shallow. The kidners are 
simple. The testes abdominal. The uterus is bicornuate. The 
placenta (in the Dugong) is non-deciduate and zonary. The um- 
bilical vesicle disappears early. The mamme are two, and pectoral, 
or rather post-axillary in position. 
The Sirenia pass their whole life in the water, being denizens of 
shallow bays, estuaries, lagoons, and large rivers, but, unlike the 
Cetacea, are not met with in the high seas, far away from the shore. 
Their food consists entirely of aquatic plants, either marine alze or 
freshwater grasses, upon which they browse beneath the surface, as 
the terrestrial herbivorous mammals do upon the green pastures on 
shore. They are generally gregarious, slow and inactive in their 
movements, mild, inoffensive, and apparently unintelligent in dis- 
position. Though occasionally found stranded by the tide or waves, 
there is no satisfactory evidence that they voluntarily leave the water 
to bask or feed on the shore. The habit of the Dugong of raising 
its round head out of the water, and carrying its young under the 
fore fin, seems to have given rise, among the imaginative early 
voyagers in the Indian Ocean, to the legendary beings, half human 
and half fish, in allusion to which the name Sirenia was bestowed by 
Illiger on the order, though certainly the face of a Dugong, when 
closely inspected, does not bear the slightest resemblance to that of 
the mermaid of romance. The specie: now existing are very few, 
and there is reason to believe that the time is not far distant when 
they will all become extinct. One species, Rhytina st-lleri, of the 
North Pacific, was totally exterminated through the azency of man 
during the last century; and the others, being valuable for their 
flesh as food, for their hides, and especially for the oil obtained from 
the thick layer of fat which lies immediately beneath their skin, 
rapidly diminish in numbers as civilised populations occupy the 
regions forming their natural habitat. The surviving species are 
confined to the tropical regions of the shores of both sides of the 
Atlantic and the great rivers which empty themselves into that 
ocean, and to the coasts of the Indian Ocean from the Red Sea to 
North Australia. In the Miocene and early Pliocene epoch 
Sirenians abounded in the seas of Europe, and their remains 
have been found in deposits of corresponding periods in North 
America. Evidence has also been discovered of the existence 
of an animal of this group in the seas at the bottom of which 
the Eocene nummulitic limestone mountain ranges of Egypt were 
deposited. * 
