The genus _Dinotherium_, sole representative of the family DINOTHERIIDAE, differs in a number of important particulars from the true Elephants. In the Elephants, if there is but a single pair of incisors, these are found in the upper jaw; in _Dinotherium_ there is apparently but a single pair, but these are implanted in the lower jaw, the symphysis of which is much prolonged and greatly bent downwards, so that the tusks emerge at right angles to the long axis of the head, and are even bent backwards. The molar teeth are five in number on each side of each jaw and are bi- or tri-lophodont, not unlike those of the Tapir. There is no cement in the valleys between the ridges of these teeth, and there is a regular succession, the premolars being two and the molars three.[146] All the teeth are in use at the same time, {232} their small size enabling them to be accommodated in the jaw together. The skull of _Dinotherium_ is lower than that of _Elephas_ or _Mastodon_. The bones of the skeleton generally are like those of _Elephas_. Though a suggestion of marsupial bones attached to the pelvis has been discredited, there is no doubt that _Dinotherium_ occupies the most primitive position among the Proboscidea; but at the same time it cannot be regarded as the ancestor of Elephants, as it is so much specialised in various ways. The incisors for one thing forbid this way of looking at the creature. It is an ancient genus found in beds of Miocene age in Europe and Asia. It is not known from America. The creature was larger than any Elephant. Eighteen feet in length has been assigned to it. The enormous weight of the lower jaw and tusks seems to argue that it was at least partially aquatic in habit, and that it may have used these tusks for grubbing up aquatic roots or for mooring itself to the bank. At first there were naturalists who considered it as an ally of the Manatee, and the skull is not unsuggestive of that of the Sirenia. _Pyrotherium_ has been referred to the Proboscidea; but our knowledge of that form is limited to a few teeth from Patagonian rocks of an uncertain age.[147] They are simple bilophodont molars, very like those of _Dinotherium_. A tusk has been found in the neighbourhood of these teeth which may possibly belong to the same animal; but it is uncertain. SUB-ORDER 7. HYRACOIDEA.