(2) The upper molars early lost the fifth cusp, and after that there was little change in the dentition, except that in the _†Merychyus_ and _†Leptauchenia_ phyla the grinding teeth became hypsodont. (3) There was great variety in the modifications of the skull, each phylum having its own peculiarities. The orbit, which was open behind in the Uinta _†Protoreodon_, was closed in the White River and all succeeding genera. In the _†Merycochœrus_ series, the skull first enlarged, with little change in proportions, then elongated the facial region, then shortened the face and so reduced the nasals as to indicate the presence of a proboscis, culminating in the grotesque, ape-like skull of _†Pronomotherium_. In the _†Leptauchenia_ phylum the skull became depressed and flattened and the face was invaded by great openings, or vacuities; the tympanic bullæ were enormously inflated and the orbits and ear-openings raised, presumably in adaptation to an amphibious mode of life. These were the extremes of change within the family; the other phyla need not be considered. (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. [Illustration: 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_