This order comprises no other living animals except the Dugongs and Manatees, which have been often placed with the true Cetaceans (Whales and Dolphins) in a common order. There is no doubt, in fact, but that the Sirenia present certain alliances to the Cetacea; and though they are to be regarded as separate orders, yet, from one point of view, they may be considered as belonging to a single section, which has been called Mutilata, from the constant absence of the hind-limbs.
The Sirenia agree with the Whales and Dolphins in their complete adaptation to an aquatic mode of life (fig. 378); especially in the presence of a powerful caudal fin, which differs from that of Fishes in being placed horizontally, and in being a mere expansion of the integuments, not supported by bony rays. The hind-limbs are wholly wanting; * and there is no sacrum. The anterior limbs (fig. 379) are converted into swimming-paddles or "flippers." The snout is fleshy and well-developed, and the nostrils are placed on its upper surface, and not on the top of the head, as in the Whales. Fleshy lips are present, and the upper one usually carries a moustache. Ears are wanting. The skin is covered with scattered bristles. The head is not disproportionately large, as in the true Whales, and is not so gradually prolonged into the body as it is in the latter. There may be only six cervical vertebrae. The teats are two in number and are "thoracic," i.e., are placed on the chest. There are no clavicles, and the digits have no more than three phalanges each. The testes are retained throughout life within the abdomen, but vesiculae seminales are present. The animal is diphyodont (Manatus), or monophyodont (Halicore); the permanent teeth consisting of molars with flattened crowns adapted for bruising vegetable food, and incisors which are present in the young animal, at any rate. In the extinct Rhytina it does not appear that there were any incisor teeth; while in all the existing genera, the front of the upper and lower jaws is provided with rough horny pads or plates.
Fig. 378. - A, Side-view of young Manatus Americanus ; B, The same viewed from above ; n Nostrils. (After Murie.)
The only existing Sirenia are the Manatees (Manatus) and the Dugongs (Halicore), often spoken of collectively as "sea-cows," and forming the family of the Manatidae.
The Manatees (fig. 380, B) are characterised by the possession of numerous broad molars, which are never all in use at one time, while there are two small upper incisors, which do not cut the gum. The tail-fin is oblong or oval in shape, and the anterior limbs (fig. 379) are furnished with nails to the four outer digits. One species (Manatus Ameri-canus) occurs on the east coast of North America, especially in the Gulf of Mexico, and another (M. Senegalensis) is found on the west coast of Africa. They are generally found in considerable numbers about the mouths of rivers and estuaries, often ranging far inland, and they appear to live entirely upon sea-weeds, aquatic plants, or the littoral vegetation. They are large, awkward animals, with a dense, rugose, hairy skin, attaining a length of from eight to ten feet as a rule, but sometimes growing to a length of nearly twenty feet.
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Fig. 379. - Fore-limb and hand of the Manatee (Manatus Americanus).
Fig. 380. - A, Side-view of the skull of the Dugong (Halicore), showing the tusk-like upper incisors; B, Side-view of the skull of Manatee (Manatus). (After Cuvier.)
The Dugongs (Halicore, fig. 380, A) have molar teeth in the young condition, but these are never all in use at one time. The molars are without enamel, and are single-rooted. Inferior incisors are present in the young animal, but are wanting in the adult. The upper jaw carries two permanent incisors, which are entirely concealed in the jaw in the females, but which increase in size in the males with the age of the animal, till they become pointed tusks. Both upper and lower jaws are strongly bent down in front, and the de-flexed portions of the jaws bear horny plates. The anterior extremities are nail-less, and the tail-fin is crescentic in shape. In their general appearance and in their habits the Dugongs differ little from the Manatees, and they are often killed and eaten. They attain a length of from eight to ten, twelve, or more feet, and are found on the coasts of the Indian Ocean and its islands, extending their range to the north coast of Australia. The bones are remarkable for their extreme density, their texture being nearly as close as ivory.
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The Manatees and Dugongs, as before said, are the only living Sirenia; but besides these there is a very singular form, the Rhytina Stelleri, which is now extinct, having been exterminated by man within a comparatively recent period. This remarkable animal was discovered about the middle of the eighteenth century in a little island (Behring's Island) off the coast of Kamtchatka. Upon this island the celebrated voyager Behring was wrecked, and he found the place inhabited by these enormous animals, which were subsequently described by M. Steller, who formed one of his party. The discovery, however, was fatal to the Rhytina. for the last appears to have been seen in the year 1768. The Rhytina was an animal of great size, measuring twenty-five to thirty-five feet in length, and twenty feet at its greatest circumference. There can hardly be said to have been any true teeth, but the jaws contained large lamelliform fibrous structures, which officiated as teeth, and may be looked upon as molars. These singular structures are not teeth, in the true sense of this term; but they are similar to the homy tuberculated plates found in the front of the mouth of the Dugong and Manatee, and the upper ones may be regarded as the equivalent of the anterior palatine pad of the Ruminants (Murie). The epidermis was extremely thick and fibrous, and hairs appear to have been wanting. There was a crescentic tail-fin, and the anterior limbs alone were present.
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As regards the distribution in time of the Sirenia, the oldest-known remains referable to the order are found in the Eocene Tertiary (Eotherhim). Of the same age is probably the interesting form described from the Tertiary deposits of Jamaica by Owen under the name of Prorastomus sirenoides. This type is remarkable as possessing upper and lower canines in addition to molar and incisor teeth. The Miocene and Pliocene deposits of Europe have yielded remains of numerous Sireni-ans belonging to the genus Halitherium, in which there are tusk-like upper incisors (as in Halicore), combined with enamelled molars (as in Manatus), and in which a rudimentary-femur is attached to the pelvis. Remains of Rhytina occur in the Post-pliocene of Siberia.