CHAPTEE VIII 



THE ORDERS SIRENIA AND CETACEA 



Order SlRENTA. 



The purely aquatic habits and fish-like form of the animals of this 

 order caused them to be formerly confounded with the Cetacea, 

 but a more intimate knowledge of their structure has shown that 

 they really belong to a widely different type of the mammalian 

 class. 



The head is rounded and not disproportionate in size as com- 

 pared with the trunk, from Avhich it is scarcely separated by any 

 externally visible constriction or neck. Nostrils valvular, separate, 

 and placed above the fore part of the obtuse truncated muzzle. 

 Eyes very small, with imperfectly formed eyelids, capable, however, 

 of contraction, and with a well-developed nictitating membrane. 

 Ear without any pinna. Mouth of small or moderate size, with 

 tumid lips beset with stiff bristles. General form of the bod)" 

 depressed, fusiform. No dorsal fin. Tail flattened and horizontally 

 expanded. Fore limbs paddle-shaped, the digits being enveloped 

 in a common cutaneous covering, on which rudiments of nails are 

 sometimes present. No trace of hind limbs in existing forms. Ex- 

 ternal surface covered with a tough, finely wrinkled, or very 

 rugose skin, naked, or with fine hairs sparsely scattered over it. 



The skeleton is remarkable for the massiveness and density of 

 most of the bones of which it is composed, especially the skull and 

 ribs, which must add to the specific gravity of these slow-moving 

 animals, and aid in keeping them to the bottom of the shallow 

 waters in which they dwell, while feeding on aquatic vegetables. 

 The skull presents many peculiarities, among which may be indicated 

 the large size and backward position of the anterior narial aperture, 

 a further modification of that met with in the Tapirs among Ungu- 

 lates, and presenting some approach to that so characteristic of the 

 Cetacea. The nasal bones are generally absent in the recent forms, 



