The Sirenia

As has been already said, nothing is known of the placentation of this small but important group of Mammalia, all the existing forms of which are aquatic in their habits, frequenting great rivers and their estuaries; and are devoid of hind-limbs, while the integument of the caudal end of the body is produced into a flattened horizontal fin. No dorsal fin is ever present. The demarcation between the head and neck is but obscurely marked, and the fore-limbs are converted into paddles, upon which only rudimentary nails are developed. Scanty bristles cover the surface of the body. The snout is fleshy and tumid, and the valvular nostrils, which are perfectly distinct from one another, are situated considerably above its termination. There is a well-developed third eyelid, the pinna of the ear is absent, and the mammae are thoracic; a circumstance which has probably not a little contributed to the origination of the myths respecting the existence of mermaids.

The Sirenia were formerly united with the Whales and Porpoises as Cetacea herbivora. But their organization differs from that of the true Cetaceans in almost every particular, while they are closely allied with the Ungulata.

The cervical vertebrae are reduced to six in one genus- Manatus. The bodies of these vertebrae are always compressed from before backward, but they are never all anchylosed together (it is rare for any of them to be thus united), and the second has a distinct odontoid process. The dorsal vertebrae have broad and depressed spines, and may be as many as seventeen or eighteen in number, while there are not more than three lumbar vertebrae; and the hindermost of these even is probably to be regarded as sacral. There are twenty or more caudal vertebrae, the terminal ones being not polygonal, but depressed, with well-developed processes.

The zygapophyses of successive vertebrae articulate together in the dorsal region; but, in the lumbar and caudal regions, the postzygapophyses disappear and the prezygapopbyses are small, and neither overlap, nor embrace, the spine of the antecedent vertebra. The posterior moiety of the spine thus acquires considerable flexibility. There is no true sacrum, the vertebra called "sacral" being only determined as such by its connection with the rudimentary pelvis. Strong subvertebral chevron-bones are placed beneath the interarticular cartilages of the caudal vertebra. The heads of the ribs articulate with the centra of all the vertebrae. The bodies of the ribs are very thick, rounded, and have a remarkably dense and laminated structure. The narrow and elongated sternum is an undivided mass of bone, and is connected by ossified sternal ribs with the anterior three pairs of vertebral ribs.

In the skull the elongated and subcylindrical form of the cranial cavity is worthy of notice, as it strongly contrasts with the form of the brain-case in the Cetacea. The supraoccipital is very large and slopes upward and forward a long way on to the upper surface of the skull; but it does not separate the parietal bones; which, as usual, unite in the sagittal suture. The frontals nre prolonged into broad supraorbital processes. The nasal bones are abortive, and, in the dry skull, the external nares are very wide, and look upward. The tympanic bone is a thick hoop, anchylosed with the periotic bones, and readily comes away from the skull with them. The zygoma is enormously stout. The premaxillae constitute a large portion of the boundary of the gape; and the lower jaw has a high ascending portion, with a large coronoid process.

The scapula has a distinct spine occupying the ordinary position. There are no clavicles. The humerus has its distal end fashioned into articular surfaces, upon which the radius and ulna are freely movable. The pollex is rudimentary, and the other digits have no more than three phalanges each.

The pelvis is rudimentary, the bones which represent the ossa innominata being connected by their proximal ends with the transverse processes of the last of the precaudal vertebrae. They are disposed vertically to the axis of the body. No trace of the hind-limbs has been observed in any of the existing Sirenia.

The premaxillary region of the palate, and the corresponding surface of the mandible, are coated with mammillated and rugose horny plates formed of hardened epithelium; and, in the extinct genus Rhytina, these plates were the only masticating organs, as there were no teeth. In Halicore (the Dugong), there are teeth which have no vertical successors, form no roots, and are devoid of enamel; while, in Manatus, there are milk-molars, and the grinding teeth are enamelled, and present crowns with double transverse ridges.

The adult Manatee has no incisors. In the Dugong there are no incisors in the mandible of the adult. The male has two tusk-like incisors which project from their sockets in the premaxillae; while, in the female, the tusks remain concealed in their alveoli.

In the foetal state, both Halicore and Manatus have incisors in the mandible as well as in the premaxillae.

The stomach is divided into two portions by a median constriction, and its cardiac end is provided with a peculiar gland. Its plyoric end, in some species, gives off two caeca.

There is a caecum at the junction of the large and small intestine. Salivary glands are well developed. The apical portion of the septum ventriculorum is deeply cleft, so that the ventricles are separated from one another through about half their extent.

There are two superior cavae and a Eustachian valve. Extensive arterial and venous retia mirabilia are developed in Manatus. In consequence of the great length of the thoracic region and the brevity of the sternum, the diaphragm takes a very unusual course, extending very obliquely from before backward, and causing the upper part of the thoracic cavity to extend posteriorly over almost the whole of the abdomen. The greatly-elongated lungs fill this part of the thoracic chamber, while the broad heart lies in its anterior and sternal portion.

The arytenoid cartilages are not prolonged as in the Cetacea. A broad and high epiglottis is capable of covering the glottis completely.

There is no third bronchus. The cutaneous muscle is largely inserted into the humerus, and the sub-caudal muscles extend forward as far as the posterior lumbar vertebrae. The chief muscles of the antibrachium and manus are present.

The male Sirenia possess vesiculae seminales. The uterus is two-horned.

There are two living genera of Sirenia—the Dugong (Halicore), which is found upon the shores of the Indian Ocean and of Australia; and the Manatee (Manatus), which is confined to the South American and African borders of the Atlantic.

A third genus, Rhytina, which had a coriaceous integument almost devoid of hair, and possessed no teeth, abounded in Behring's Straits less than a century ago. It is now altogether extinct.

The Miocene genus, Halitherium, appears to have possessed distinct, though small, hind-limbs.