Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/413

Rh CETACEA.] of tvo small islands in the North Pacific 1, Bfhring s and the adjacent Copper Island, on the former of which it was discovered by the ill-fated navigator whose name the island bears, when, with his accomplished companion, the German naturalist Steller, he was wrecked upon it in 1741. Twenty-seven years afterwards (1768), as is commonly supposed, the last of the race was killed, 1 and its very existence would have been unknown to science but for the interesting account of its anatomy and habits left by Steller, and the few more or less perfect skeletons which have recently rewarded the researches carried on in the frozen soil of the islands around which it dwelt. There is no evidence at present of its having inhab ited any other coasts than those of the islands just named, though it can hardly be supposed that its range was always so restricted. &quot;When first discovered it was extremely numerous in the shallow bays round Behring s Island, finding abundant nutriment in the large laminarite growing in the sea. Its extirpation is entirely due to the Russian hunters and traders who followed upon the track of the explorers, and who, upon Steller s suggestion, lived upon the rlesh of the great Sea-cows. Its restricted distribution, large size, inactive habits, fearlessness of man, and even its affec tionate disposition towards its own kind when wounded or iu distress, all contributed to accelerate its final extinction. EXTINCT SIUEXIA. The Miocene and early Pliocene seas of Europe abounded in Sirenians, to which the generic name of JIalitkeriiim was given by Kaup. They had largo tusk-like incisors in the upper jaw, as in the existing Dugongs, though not so greatly developed. Their molar teeth were or, anteriorly simple and single-rooted, posteriorly those above with three and those below with two roots, and with enamelled and tuberculated or ridged crowns, in all which respects they more resemble those of the Manatee than of the Dugong. The anterior molars were deciduous. Some species at least had nasal bones, short, broad, but normal in position, whereas in all the existing genera these bones are quite rudimentary. An other and still more important evidence of conformity to the general mammalian type is the better development of the pelvic bone, and the presence of a small styliform femur articulated to the acetabulum, although no traces of any other part of the limb have been dis covered. These ancient Sirenians were thus, in dental, cranial, and other osteological characters, less specialized than are either of the existing species, and, if the intermediate links could be discovered, might well be looked upon as ancestral forms from which the latter have been derived, but at present the transitional conditions have not been detected. So far as is yet known, when changes in the physical conditions of the European seas rendered them unfitted to be the habitation of Sirenians, the Halitherium type still prevailed. If the existing Dugongs and Manatees are descended from it, their evolution must have taken place during the Pliocene and Pleistocene epochs, the one in seas to the east, the other to the west of the African continent, which has long formed a barrier to their intercommunication. Halitherium remains have been found in many parts of Germany, especially near Darmstadt, also in France, Italy, Belgium, Malta, the isthmus of Suez, &c. Until lately none were known from England, probably owing to the absence of beds of an age corresponding to those in which they are found on the European continent ; but recently a skull and several teeth have been detected among the rolled debris of Miocene formations, out of which the Red Crag of Suffolk is partially composed. The species are not yet satisfactorily characterized. Some of them appear to have attained a larger size than the existing Manatee or Dugong. One of these from the Pliocene of Italy and France, having but f molar teeth, has been separated generically under the name of Felsinotherium by Capellini, by whom it has been fully described. A portion of a skull found in Belgium has been named Crassithcriuni by Van Bencden; and some compressed teeth, some what similar to but larger than those of the Dugong, discovered in the department of Lot-et-Garonne, France, gave origin to the genus Rytiodus of E. Lartet. Of this more complete remains have recently been described by Delfortrie. The rostrum is more elong ated than in Halitherium, but the skull is otherwise very similar, as are the molar teeth. The incisors are very large, exserted, strongly compressed, almost sabre-like, rounded on the upper or anterior surface, sharp below, concave on the external and convex on the inner side, and transversely striated. PacTiy acanthus of Brandt, from the Vienna basin, is also, accord ing to Van Beneden, another form of Sirenia, of which, however, the skull is not known. In various Miocene and perhaps Eocene marine formations of the United States of America remains of Sirenians have been found, but mostly in such a fragmentary con dition that they afford at present little evidence of the early history of the group in that country. A more satisfactory discovery is that 1 Nordenskiold, during his recent voyage in the &quot; Vega,&quot; obtained some information from natives of Behring s Island which led him to believe that a few individuals may have survived to a much later date, even to 1854. 391 of a nearly complete skull and some bones from a limestone Tertiary formation in Jamaica. It is of smaller size than the Manatee, and as far as the teeth are concerned, of a still more generalized character than Halitherium, the dentition being apparently i f, c, p , ?&amp;gt;if=48. The incisors are small, not developed into tusks ; the canines (wanting in all existing Sirenians) are rather larger than the incisors, judging by the sockets; and the molars are bilophodont, and covered with enamel. It has been described by Professor Owen under the name of Prorastomus sirenoides. Unfortunately we have no knowledge of the geological antiquity of the formation in which it was embedded. Lastly must be mentioned the Eothcrium egyptiacum, Owen, founded on the cast of a brain, with a small quantity of surrounding bone, discovered in the nummulitic lime stone of Eocene age of the Mokattam Hills, near Cairo. The brain is narrower than in Manatus, and resembles that of Haliilierium. This is of interest as the most ancient known evidence of any Sirenian whose age has been geologically determined. The few facts as yet collected relating to the former history of the Sirenia leave us as much in the dark as to the origin and affinities of this peculiar group of animals as we were when we only knew the living members. They lend no countenance to their association with the Cctacea, and on the other hand their supposed affinity with the Ungulata, so much favoured by modern zoologists, receives no very material support from them. Bibliography of Sirenia. J. F. Brandt, Symbolie Sirenologicx, St Petersburg, 3 fasciculi, 1846-61-68, an exhaustive account of the anatomy, affinities, and literature of the group, with copious illustrations of the osteology of Ji/iytina. Anatomy of Duyong: Everard Home, Phil. Trans., 1820, p. 315; Owen, Proc. Zuol. Soc., 1838, p. 29. Manatee : Vrolik, liijdr. tot dr. Dierkunde, 1851 ; Murie, Trans. Zool. Soc. Land., vol. viii. p. 127, 1870, and vol. xi. p. 19, 1880; Garrod, ibid., vol. x. p. 137, 1875. .Extinct Sirenia : Gervais, Journal de Zoologiv, torn. i. p. 332, 1872. ORDER CETACEA. This is perhaps the most distinctly circumscribed and natural of all the larger groups into which the class is divided. The external form is Fish-like, the body being fusiform, passing anteriorly into the head without any distinct con striction or neck, and posteriorly tapering off gradually towards the extremity of the tail, which is provided with a pair of lateral, pointed expansions of ekin supported by dense fibrous tissue, called &quot;flukes,&quot; forming together a horizontally placed triangular propelling organ, notched in the middle line behind. The head is generally large, in some species attaining to even more than one-third of the entire length of the animal, and the aperture of the mouth is always wide, and bounded by stiff immobile lips. The fore limbs are reduced to the condition of flattened ovoid paddles, encased in a continuous integument, showing no external sign of division into fore arm and manus, or of separate digits, and without any trace of nails. There are no signs of hind limbs visible externally. The general surface of the skin is smooth and glistening, and devoid of hair, although in many species there are a few fine bristles in the neighbourhood of the mouth, which may persist through life or be present only in the young state. Immediately beneath the skin, and intimately connected with it, is a thick layer of fat, held together by a dense mesh of areolar tissue, constituting the &quot; blubber,&quot; which serves the purpose of the hairy covering of other mammals in retaining the heat of the body. In nearly all species a compressed median dorsal tegumentary fin is present. The eye is small, and is not provided with a nictitating membrane or true lacrymal apparatus. ^ The external auditory meatus is a very minute aperture in the skin situated at a short distance behind the eye, and there is no vestige of a pinna. The nostrils open separately^ by a single crepcentic valvular aperture, not at the extremity of the snout, but near the vertex. The bones generally are spongy in texture, the cavities being filled with oil. In the vertebral column, the cervical region is remarkably short and immobile, and the vertebrae, originally always seven in number, are in many species more or less fused together into a solid mass. The odontoid process of the axis, when that bone is free, is usually very obtuse, or even obsolete. None of the vertebras arc united