Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/412

390 M A M M A L I A [SIRENIA. in Mantitus. The apex of the heart is deeply cleft between the ventricles. The principal arteries form very extensive and complex rctia mirabilia. The lungs are remarkably long and narrow, as, owing to the very oblique position of the diaphragm, the thoracic cavity extends far back over the abdomen. The epiglottis and arytenoid cartilages of the larynx do not form a tubular prolongation as in the Cetacea. The brain is of comparatively small size, and the convolutions "on the surface of the cerebrum few and shallow. The kidneys are simple. Testes abdominal. The uterus is bicornuate. The placenta (in the Dugong) is non-deciduate and diffuse, the villi being scattered gene rally over the surface of the chorion except at the poles. The umbilical vesicle disappears early. The mammae are two, and pectoral or rather post-axillary in position. The Sirenia pass their whole life in the water, being denizens of shallow bays, estuaries, lagoons, and large rivers, but unlike the Cetacea are not met with in the high seas, far away from the shore. Their food consists entirely of aquatic plants, either marine algae or freshwater grasses, upon which they browse beneath the surface, as the terrestrial herbivorous mammals do upon the green pastures on shore. They are generally gregarious, slow and inactive in their movements, mild, inoffensive, and apparently unintelligent in disposition. Though occasionally found stranded by the tide or waves, there is no satisfactory evidence that they voluntarily leave the water to bask or feed on the shore. The habit of the Dugong of raising its round Lead out of water, and carrying its young under the fore fin, seems to have given rise, among the imaginative early voyagers in the Indian Ocean, to the legendary beings, half human and half fish, in allusion to which the name Sirenia was bestowed by Illiger on the order, though certainly the face of a Dugong, when closely inspected, does not bear the slightest resemblance to that of the mermaid of romance. The species now existing are very few, and there is reason to believe that the time is not far distant when they will all become extinct. One species, Rhytina stelleri, of the North Pacific, was totally exterminated through the agency of man during the last century; and the others, being valuable for their flesh as food, for their hides, and especially for the oil obtained from the thick layer of fat which lies immediately beneath their skin, rapidly diminish in numbers as civilized populations occupy the regions forming their natural habitat. The surviving species are confined to the tropical regions of the shores of both sides of the Atlantic and the great rivers which empty themselves into that ocean, and to the coasts of the Indian Ocean from the Red Sea to North Australia. In the Miocene and early Pliocene epoch Sirenians abounded in the seas of Europe, and their remains have been found in deposits of corresponding periods of North America. Evidence has also been discovered of the existence of an animal of the group in the seas at the bottom of which the Eocene nummulitic limestone mountain ranges of Egypt were deposited. The existing genera present such well-marked distin guishing characters that, if they alone were known, they might be placed in separate families ; but, as in so many similar cases, our knowledge of the extinct forms, imperfect as it is, goes far to bridge over the distinction between them. Manatus. Incisors , rudimentary, concealed beneath the horny oral plates, and disappearing before maturity. Molars -J-}-, but rarely more than present at one time ; the anterior teeth falling before the posterior come into use ; similar in characters from beginning to end of the series ; with square, enamelled crowns, the grinding surface raised into tuberculated transverse ridges. The upper teeth with two ridges and three roots, the lower teeth with an additional (posterior) ridge or talon and tvo roots. The cer vical vertebra} present the remarkable anomaly of being reduced to six in number, the usual vertebral formula being 06, D 15-1 8, L and C 25-29. Rostrum of the skull, formed by the union of the premaxillae in front of the anterior narial aperture, shorter than the length of the aperture and scarcely deflected from the basi-cranial axis. Tail entire, rounded or shovel-shaped. Rudimentary nails on the fore limbs. Caecum bifid. Habitat the shores of, and the great rivers which empty themselves into, the Atlantic within the tropics. The American and African forms are generally considered to be distinct species (M. austmlis and M, senegalensis), though they differ but little from each other in anatomical characters and in habits. They are rather fluviatile than marine, ascending large rivers almost to their sources. See MANATEE. JIalicorc. Jn the upper jaw a pair of large, nearly straight, tusk- like incisors, directed downwards and forwards, partially coated with enamel. In the male they have persistent pulps, and bevelled cutting edges, which project a short distance from the mouth, but in the female, though they remain through life in the alveolar cavity, they are not exserted, and, the pulp cavity being tilled with osteodentine, they soon cease to grow (as in the female Narwhal). In the young there is also a second small deciduous incisor on each side above. At this age there are also beneath the horny plate which covers the anterior portion of the mandible four pairs of slender conical teeth lodged in wide alveolar depressions. These become absorbed before the animal reaches maturity. The molars are usually f, sometimes f, altogether, but not all in place at once, as the first falls before the last rises above the gum ; they are more or less cylindrical in section, except the last, which is compressed and grooved laterally, without distinction into crown and Voot, increasing in size from before backwards, with persistent pulps and no enamel. The summits of the crowns are tuberculated before wear- FIG. 41. American Manatee (Manatus australis), from life. Proc. Zoo!. Soc., 1881, p. 407. ing, afterwards flattened or slightly concave. Skull with rostrum formed by the union of the premaxillee in front of the narial aperture, longer than the aperture itself, bending downwards at a right angle with the basi-cranial axis, and enclosing the sockets of the large incisor tusks. Anterior part of the lower jaw bent down in a corresponding manner. Vertebrae: 07, D18-19, Land C 30. Tail broadly notched in the middle line, and with two pointed lateral lobes. No nails on the fore limbs. Caecum single. The Dugongs are more distinctly marine in their habits than the Manatees, feeding chiefly on sea-water alg;e. They inhabit the shallow bays and creeks of the Red Sea, east coast of Africa, Ceylon, islands of the Bay of Bengal and the Indo-Malayan Archipelago (including the Philippines), and the north coast of Australia, ranging from Barrow Reefs on thu west to Moreton Bay on the east. Though the distinctive characters are not very obvious, they have been divided into three species, according to the localities which they respectively inhabit : //. tabernaculi from the Red Sea, H. dugoi/g from the Indian seas, and JL australis from Australia. The last-named has lately been the object of a regular "fishery," chiefly on account of its oil, which is peculiarly clear, limpid, and free from disagreeable smell, and is said to have the same medical properties as cod-liver oil. Although often stated in books to attain the length of 20 feet when adult, there does not appear to be any evidence from actual specimens in museums that Dugongs ever reach half that size, 8 feet being the common length of adult animals. Rhytina. No teeth, their place being supplied functionally by the dense, strongly-ridged, horny oral plates. Premaxillary rostrum about as long as the anterior narial aperture, and moderately deflected. Vertebra: C 7, D 19, L and C 34-37. Head very small in proportion to the body. Tail with two lateral pointed lobes. Pectoral limbs small and truncated. Skin naked and covered with a very thick, hard, rugged bark-like epidermis. Stomach without c;"cal appendages to the pyloric cavity. Ctccum simple. Only one species of this genus is known, R. stdleri, the Northern Sea-cow, by far the largest animal of the order, attaining the length

of 20 to 25 feet. It was formerly an inhabitant of tho shores