(4) Unfortunately, nothing is yet known of the skeleton of †Bathyopsis and †Elachoceras, so that it is not practicable to follow out all the stages of skeletal modification, though the general course of development is sufficiently plain. The neck did not change greatly, except to become very strong and heavy and to grow shorter proportionately as the skull was lengthened. The body remained long throughout the series, but gained greatly in bulk, as the stature of the animal increased.
(5) The limb-bones lost their primitive character, such as the epicondylar foramen of the humerus and the third trochanter of the femur, and then, with the great increase of the weight to be supported, the marrow-cavities were filled with spongy bone and the hip-bones increased enormously in width; the femur lost its cylindrical shape and was flattened antero-posteriorly, which gave it a very elephantine appearance. None of the limb-bones was suppressed or greatly reduced in size, nor was there any coössification between them.
(6) The feet early gained their definitive character; at no time was there any loss of digits, but the originally divided toes were, in the genera of the Wasatch and subsequent stages, united into the columnar foot, and the hoofs were reduced from their primitively pointed shape to nodular form.
As in the Proboscidea, therefore, there was comparatively little change in the skeleton after the massive and bulky proportions had been acquired, but great and continual modification of the skull. At the time when the †Amblypoda finally disappeared, no ungulate had acquired the hypsodont dentition. Had the group survived till the middle Miocene, a time when the spread of grassy plains so profoundly affected the feeding habits of many herbivorous mammals, the high-crowned teeth might have been developed in them also, and this, in turn, would have produced other changes in the skull, making closer the parallel with the Proboscidea.
In conclusion, a few words may be said concerning the geographical distribution of the †Amblypoda. In the Paleocene the only known representatives of the order were those of North America, but the †coryphodonts of the lower Eocene migrated to the Old World; indeed, the genus †Coryphodon was first described and named from English specimens, but there were no such abundance and variety of these animals in Europe as there were in the western United States. The †uintatheres were strictly North American in distribution and no member of the suborder has ever been found outside of this continent. Animals referred to the †Amblypoda by some authorities have been obtained in the Oligocene and Miocene of South America, but the assignment has been made upon insufficient evidence. (See [p. 508].)
Order †Condylarthra
The †Condylarthra were a group of exceedingly primitive ungulates, which served to connect the hoofed and clawed mammals in quite an intimate manner. So few indeed were the distinctively ungulate characters which they had acquired, that it is still premature to make any positive statements regarding their geographical distribution, because unusually well-preserved specimens are required to make sure of their presence in any particular region. Concerning North America there is no room for question, and there is hardly any doubt that they existed in the Paleocene of Europe. The South American remains which have been referred to this order may very well prove eventually to belong to it properly, but until both feet and skulls have been obtained in unequivocal association, the reference can be only tentative. In North America they ranged through the Paleocene and lower Eocene, but are not known from any subsequent formation, and even in the Wind River only a few stragglers survived.
The principal American families and genera are as follows: