» CHAPTER VIE 
THE ORDERS SIRENIA AND CETACEA 
Order STRENIA. 
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 which 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 body 
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 verr 
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, 
