166 MAMMALIA ORDER VILSIRENIA. 



instead of being laterally compressed, is flattened from above downwards. 

 The head, again, is of an ordinary type, being relatively small in comparison 

 to the body, with the nostrils placed at the extremity of the muzzle, and the 

 summit rounded. To meet the exigencies of an aquatic life, external ears 

 are, however, wanting, and the nostrils are capable of being closed at will 

 by means of tightly- fitting valves, so as to prevent the ingress of water. In 

 no case is the back furnished with a fin ; and the fore flippers may retain 

 traces of nails, although the whole of their toes are enveloped in a common 

 skin, while the skeleton of these parts contains no more than the normal 

 complement of bones to the toes that is to say, each toe, except the first 

 (where there are but two), has three joints. The backbone, or spinal column, 

 differs from that of land mammals, in that none of the vertebrae in the 

 region of the haunches are welded together to form a sacrum, such a struc- 

 ture being unnecessary in an animal devoid of hind limbs, while its presence 

 would hinder the supple movements of the body so essential in purely aquatic 

 creatures. From those of whales and porpoises, the vertebrae of existing 

 sirenians differ by the absence of thin bony plates at the extremity of each 

 end of the body, or lower portion of the same ; although these were well 

 developed in certain extinct members of the order. The manati is remark- 

 able for the circumstance that the seven vertebrse of the neck, which are so 

 remarkably constant in the class, are reduced to six. Neither of the genera 

 have collar-bones, or clavicles, but in all the members of the group the bones 

 of the skeleton are remarkable for the extreme solidity and denseness of 

 their structure. Great variability is displayed in regard to the dentition ; the 

 recently extinct species from Behring Strait having no teeth at all ; whereas 

 in the manatis there is a very full series of masticating organs, which present 

 a certain resemblance to the molars of the hippopotamus. In no case do the 

 teeth resemble those of the toothed cetaceans ; and although in the recent 

 forms milk-teeth are not developed, rudiments of these have been detected, 

 and in certain extinct kinds such teeth were functional. Similarly, nasal 

 bones, or those roofing the cavity of the nose in ordinary mammals, have 

 disappeared in the existing representatives of the group, although they were 

 well developed in the earlier forms. From these peculiarities it is evident 

 that the existing sirenians are specialised creatures evidently derived from 

 land mammals of a more ordinary type. To fit them for long periods of sub- 

 mergence, both manatis and dugongs have the lungs much elongated, and 

 extending almost the whole length of the back ; while the blood-vessels are 

 expanded in certain parts of the body into net-like structures, known as 

 retia mirabilia, in which pure blood can be held back for a considerable time, 

 instead of immediately passing onwards to the heart. 



As regards their mode of life, the sirenians are inhabitants of large rivers, 

 estuaries, sheltered bays, and shallow seas generally ; never ploughing 

 through the water of the open ocean in the manner so characteristic of the 

 whales and porpoises ; and seldom, indeed, wandering out of sight of shore. 

 Moreover, instead of having the carnivorous habits so characteristic of the 

 cetaceans, all the sirenians are exclusively vegetable feeders ; tearing up the 

 sea-weed and tangle when in the ocean, and other aquatic plants when they 

 frequent fresh- waters, and masticating them after the manner of a pig either 

 by means of their powerful molar teeth, or, when these are wanting, with 

 the aid of horny plates which then invest the surface of the mouth. Both 

 manatis and dugongs produce but a single offspring at a birth, which is 

 tended with great care and affection by the mother. When suckling, the 



