AQUATIC MAMMALS 



The sirenians or sea cows are exclusively aquatic and inhabit rivers 

 and coastal waters where aquatic plants and algae abound. The fore 

 limbs are paddle shaped, there are no external hind limbs, and the tail 

 is excessively flattened vertically and correspondingly expanded hori- 

 zontally. The nostrils are considerably elevated, the eyes are small and 

 are furnished with nictitating membranes, and there are no external 

 ears. The skin is rugose and almost hairless. The mammae are lateral 

 and postaxillary and the testes are abdominal. All the bones are ex- 

 ceedingly dense and heavy, the external nares are high upon the rostrum 

 and the nasal bones are usually absent (present in the manati). The 

 neck is shortened, there is no clavicle or sacrum and the pelvis is ex- 

 tremely vestigeal. The phalanges are moderately flattened. The lungs 

 are long and shallow (vertically) and the diaphragm is almost hori- 

 zontal and extends far back. There are very extensive retia mirabilia. 

 Swimming is accomplished exclusively by the tail and individuals (at 

 least of the manati) habitually rest with the back excessively curved 

 and the tail tip touching the bottom (see figure 7). The pectoral 

 limbs are used in feeding and are more mobile than in the Cetacea. 



Trichechidae 



Trichechus — the manati inhabits rivers in parts of eastern tropical 

 America and locally in tropical Africa. Nasal bones are present and 

 there are but six cervical vertebrae. The nostrils are not quite so ele- 

 vated as in the Halicoridae, the tail is shovel-shaped, and vestigeal 

 nails upon the flippers are usually present (absent in the form inun- 

 guis) . 



Halicoridae 



Halicore — the dugong is found in shallow bays and rivers locally 

 from the Red Sea to Australia, and feeds chiefly upon marine algae. 

 The nares, both externally and in the skull, are more elevated than in 

 the manati, and there are no nasal bones. There are seven cervical 

 vertebrae. The tail is notched centrally and the lateral tips are quite 

 pointed rather than rounded. Nails are absent. 



Hydrodamahdae 



Hydrodmualis — Steller's sea cow inhabited the vicinity of Bering and 

 Copper islands in the North Pacific, where they fed upon marine algae, 

 but they were exterminated more than one hundred and fifty years ago 

 [40] 



