(4) At an early stage the digits were reduced from five to four, first in the pes and then in the manus, and there reduction ceased; though in †Merychyus, especially in the upper Miocene species, the lateral digits were very slender and, had this series survived, it would probably have led to didactyl forms.
Fig. 204.—Left manus of †oreodonts. A, †Merycoidodon culbertsoni, White River. B, †Merycochœrus proprius, upper Miocene.
In other respects there was very little difference in the skeletons of the various phyla and herein lies the peculiarity in the history of the family, great variety in the form of the skull, and, relatively speaking, hardly any change in the body, limbs or feet. In the horses, rhinoceroses and †titanotheres the modifications of the successive genera affected all parts of the structure, but in the †oreodonts, except for the loss of one digit in manus and pes and variations in the length of the tail, the skeletons of the latest genera did not differ in any important respect from those of the earliest. Such a combination of mutability and plasticity in the skull with extreme conservatism in the remainder of the bony structure is an exception to the usual mode of development, though something of the same sort has already been pointed out in the case of the tapirs ([p. 325]) and will recur in that of the elephants ([Chap. X]).
5. †Agriochœridæ. †Agriochœrids
This family, one of the strangest and most aberrant of ungulate groups, was very closely allied to the †oreodonts and by many authorities is included in the same family. The history of the successive steps of discovery, by which the structure of these extraordinary animals was gradually made plain, is much the same as in the case of the even more peculiar perissodactyl family of the †chalicotheres ([p. 356]). The various parts, found scattered and at long intervals of time, had been referred to no less than three different mammalian orders! for, until the discovery of †chalicothere skeletons gave the clue, no one imagined that such discordant parts could belong to the same animal.
Fig. 205.—Skull of †Agriochœrus latifrons, White River. (After Wortman.)
