This order comprises certain extraordinary extinct Mammals from the Eocene of North America, which are regarded by Prof. Cope as an aberrant group of Ungulates, whilst Prof. Marsh considers them as a distinct order intermediate between the Perissodactyle Ungulates and the Proboscidea.
The members of this order are all of gigantic dimensions, and of massive construction. Both the hind-feet and fore-feet possessed five well-developed toes. The nasal bones were elongated, and do not seem to have supported a proboscis. The cranium carries three pairs of horn - cores, which were probably enveloped in horny sheaths. There are no upper incisors, and the upper canines have the form of long tusks directed downwards. (These characters are taken from Dinoceras, the best-known genus of the group.) The order is distinguished from the Proboscidea by the absence of upper incisors, the presence of canines, the possession of three pairs of horn-cores, and the absence of a proboscis.
In Dinoceras itself, which may be taken as the type of the group, we have a large animal equal in dimensions to the living Elephants, which it resembles also in the osteology of its limbs, in most essential respects. It is in the skull (fig. 413) and dentition, however, that the most striking peculiarities of Dinoceras are to be found. As regards the dentition, the front of the upper jaw was destitute of incisors, and probably carried a palatine pad, but there were two very large canines in the form of tusks directed perpendicularly downwards; and there was also a series of six small grinders on each side. In the lower jaw are six incisors, small canines, and twelve praemolars and molars, six on each side. The dental formula is thus:
0 - 0
1 - 1
3 - 3
3 - 3
3 - 3
1 - 1
3 - 3
3 - 3
Superiorly each maxillary bone carried a well-developed process, probably of the nature of a horn-core. The nasals support two similar but smaller horn-cores; and the frontals are developed behind into two larger bony projections most probably also of the nature of horn-cores. The animal thus possessed three pairs of horns, one carried by the upper jawbones, one by the nasals, and one by the frontal bones; though it is possible that some of these cores were simply covered by a callous integument. The nasal bones are long, and there is no evidence of any proboscis. The limbs are short, the forelegs shorter than the hind-legs; and the femur was not provided with a third trochanter. The tail is short and slender, and the ribs are furnished with rudimentary uncinate processes. As regards the mental powers of Dinoceras, Prof. Marsh remarks : "The brain-cavity of Dinoceras is perhaps the most remarkable feature in this remarkable genus. It proves conclusively that the brain was proportionately smaller than in any other known Mammal, recent or fossil, and even less than in some reptiles. It is, in fact, the most reptilian brain in any known Mammal. In D. mirabile, the entire brain was actually so diminutive that it could apparently have been drawn through the neural canal of all the praesacral vertebrae, certainly through the cervicals and lumbars."
Fig. 413. - Skull of Dinoceras mirabile, after Marsh. From the Eocene Tertiary.
The chief genera which are included amongst the Dinocerata by Marsh are Dinoceras, Tinoceras, and Uintatherium. All the remains of this singular group which have hitherto been brought to light, are from the Eocene rocks of North America.