The head is rounded and not disproportionate in size as compared 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. External 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 Ungulates, and presenting some approach to that so characteristic of the Cetacea. The nasal bones are generally absent in the recent forms, or are only found in a most rudimentary condition, attached to the edge of the frontals, far away from the middle line; but in some at least of the extinct species these bones, though small in size, are normal in situation and relations. In very few other respects does the skull present any resemblance to that of the Cetacea. In the spinal column of existing forms none of the vertebræ are united together to form a sacrum, and the flat ends of the bodies do not ossify separately, so as to form disc-like epiphyses in the young state, as in nearly all other mammals; traces of epiphyses have, however, been recently detected in _Manatus_, and they were fully developed in _Halitherium_ and other fossil forms. The anterior caudal vertebræ have well-developed chevron bones. In one genus (_Manatus_) there are only six cervical vertebræ. There are no clavicles. The humerus has a small but distinct trochlear articulation at the elbow-joint. The two bones of the forearm are about equally developed, and generally ankylosed together at both extremities. The carpus is short and broad, and the digits five in number, with moderately elongated and flattened phalanges, which are never increased in number beyond the limit usual in the Mammalia. The pelvis is extremely rudimentary, consisting of a pair of bones suspended at some distance from the vertebral column. In no existing species is