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Sirenia (sea cows, mermaids; cohort Ferungulata, superorder Paenungulata) An order of herbivorous ungulates that have adapted to a fully aquatic life. The body is streamlined, thick-skinned, and has a thick layer of blubber. The fore limbs are pentadactyl, the digits are joined, forming large paddles, but considerable movement is possible at the wrist and elbow and in the digits. There are no hind limbs, and the pelvic girdle is vestigial. The tail is modified to form a horizontal fin. The ribs are numerous and form a barrel-shaped cage extending far back in the body, and the diaphragm is oblique, so the lungs are large. The bones are heavy, with little marrow (pachyostosis). The skull is large, but the brain case is small. The young are born in the water and suckle from pectoral teats. Fossil sirenians are known from the Eocene, in Egypt and Jamaica. Probably they appeared first in Africa, from the stock that also gave rise to the proboscideans; and the two surviving families, Dugongidae and Trichechidae, diverged possibly before the end of the Eocene and so are different in many ways from one another.