Proboscidea.—The evolution and structure of the elephants (q.v.) are considered in the article PROBOSCIDEA, and further information can be found in H. F. Osborn's Monograph of this group. As to the remote origin of the Proboscidea, the fact that Arsinoitherium, in spite of its wholly different skull and dentition, shows so many curiously detailed resemblances to the Proboscidea in its limbs and backbone, lends some support to C. W. Andrews' view that the group of "Subungulata," consisting of the Probo scidea, Hyracoidea, Embrithopoda, Amblypoda, may after all be a more or less natural assemblage of ungulates. In fact we may even advance the tentative hypothesis that some such small Lower Eocene condylarth as Hyopsodus walcottianus (described by Matthew and Granger), with short spreading feet, reduced canines, slightly procumbent incisors and bunodont molars, would have been an ideal starting-point for the entire subungulate series, including also the South American Pyrotheria and perhaps even the Sirenia.
Sirenia (q.v.).—If "common sense" and superficial appear ances were trustworthy, these surprisingly whale-like mammals would still be classified with the Cetacea (q.v.) as they were by all early naturalists. But De Blainville as far back as 1816 classified them as "ongulogrades anomaux pour nager"—anomal ous ungulates adapted for swimming; in his later classification he brigaded them with the Proboscidea under the term"Gravigrades." Andrews in his description of Eosiren (the oldest known sirenian, from the Upper Eocene of the Fayum, Egypt) pointed out a number of significant features in which the skull of Eosiren re sembled that of Moeritherium, the oldest and most primitive known proboscidean from the same formation. He also cited a number of curious anatomical details in which even the modern Sirenia agree with the elephants in spite of the enormous difference in their external appearance and mode of life. On the other hand, R. Lydekker pointed out that the unworn molar teeth of certain extinct sirenians were curiously like those of certain extinct artiodactyls (Merycopotamus) and that this fact suggested the derivation of the Sirenia from very early Eocene artiodactyls; but one might equally say that the molar teeth of another extinct sirenian (Miosiren) suggest those of the Eocene rodent Ischyro mys and that the order Sirenia had therefore been derived from primitive rodents. Either of these views would be hard, in the present meagre state of our knowledge of the subject, to disprove; but neither has nearly as much positive evidence in its favour as the view of De Blainville and Andrews that the Sirenia are an aquatic specialization from the Proboscidean stem.
Be that as it may, however, by the time of the Lower Oligocene Eosiren was already definitely a sirenian in its dentition, skull and locomotor skeleton. Thereafter during the Oligocene, Miocene, Pliocene and Pleistocene epochs the changes in the skull and skeleton were comparatively slight and unimportant. The middle and late Tertiary sirenians (such as Halitherium and Metaxy therium) were for the most part considerably larger than the modern manatees and dugongs (qq.v.) and had a large vertically placed pair of upper tusks, which have been retained by the dugongs but lost by the manatees.
The Sirenia are whale-like in their torpedo-like bodies and horizontal tail flukes, in the complete absence of external hind limbs and in their flipper-like forelimbs, which however still retain external nails or vestigial hoofs. They differ markedly from typical cetaceans in their relatively small heads, truncate flattened snouts with transversely expanded upper lips covered with very large bristles. The molar teeth are two-ridged,—wholly unlike the conical teeth of toothed cetaceans. Their tusks when present are situated at the end of the muzzle and directed downward. They differ profoundly from cetaceans in their food and feeding habits, being the herbivores of the coasts and estuaries, whereas at least the typical cetaceans are essentially carnivores of the open seas. In their internal anatomy the sirenians likewise differ widely from cetaceans : the musculature of the forelimbs is less profoundly modified for aquatic life, the brain has a much less complexly convoluted surface and the digestive tract recalls that of the ruminants. The skeleton of sirenians is peculiarly massive and dense, the swollen, heavy ribs serving apparently as ballast to keep these voluminous, gas-filled bodies below the surface.
Most of the many known extinct types of sirenians conform in essentials to the dugong type, but one extinct family, the Mio cene Desmostylidae, is widely different from the rest. In these the large upper tusks almost suggest the earliest proboscidean types but the most remarkable peculiarity is found in the molar teeth, each of which consists of a closely packed cluster of cylindrical columns of circular cross-section.