Fig. 250.—Left pes of †Diadiaphorus, from specimens in Princeton University and the American Museum. Cal., calcaneum. As., astragalus. N., navicular. Cn. 3, external cuneiform. Cb., cuboid.
In still another respect the feet of †Diadiaphorus deviated markedly from those of the horses, viz. in the great proportionate length of the phalanges, especially of the first one, and the shortness of the metapodials, the three phalanges of the median digit together exceeding in length the metacarpal or metatarsal, while in the horses this proportion is reversed. The skull of this genus was short, deep and with an anterior taper; it had a long sagittal crest, but a brain-chamber of good capacity, considering its geological date. The nasals were quite short, though the degree of shortening was not such as to suggest the existence of a proboscis. In general appearance the skull recalls that of one of the larger †oreodonts ([p. 372]) of the North American Oligocene.
To the genus †Proterotherium, the type of the family, belonged a great number of Santa Cruz species, for at that time the genus was in a state of most vigorous development and the species were so variable that satisfactory discrimination of them is exceedingly difficult. They were all much smaller and slighter animals than the species of †Diadiaphorus, but did not differ from them in any important structural character. The skull in this genus closely resembled that of the one last named, save for its smaller size and lighter and more slender proportions; the nasal bones were considerably longer and the occiput was somewhat wider.
Fig. 251.—Skull of †Thoatherium, Santa Cruz. Princeton University Museum.
A more isolated position was held by the genus †Thoatherium, which was very clearly demarcated from all of the other genera of the family. Its species were the smallest of the commoner Santa Cruz members of the order and were of very light and graceful form. The dental formula was the same as in the other genera, but there were no tusks; the single upper and two lower incisors were of nearly the same size and simple, chisel-like form. The upper molars had the same elements as in the preceding genera, but somewhat differently connected, the two internal cusps and the anterior intermediate cuspule being united into a nearly longitudinal ridge. The skull was light, slender and pointed; the nasals were shortened, though less than in †Diadiaphorus; the sagittal crest was shorter than in the latter and the occiput was far narrower. The neck was short, the body of moderate length and the tail short. The limbs and especially the feet were proportionately more elongate and slender than in any other known genus of the family, giving quite a stilted appearance to the skeleton. The fore-arm bones were not coössified, but the ulna was much more reduced than in any of the other genera of the family, and the same is true of the fibula, which, though very slender, showed no tendency to unite with the tibia. The limb-bones, especially the femur, had a decided resemblance to those of †Mesohippus, the lower Oligocene tridactyl horse of North America, with the smaller species of which, †M. bairdi, †Thoatherium agreed well in size. Most remarkable of all were the feet, which were more strictly monodactyl than those of any other known mammal. The single functional digit, the third, had on each side of its upper end a very small, scale-like nodule of bone, the last vestiges of the lateral digits, corresponding to the immensely larger splints of the horse. Despite the unrivalled completeness of digital reduction which †Thoatherium displayed, the mode of reduction was inadaptive and the rudimentary metapodials retained the same carpal and tarsal connections that they originally had in the pentadactyl manus, a very great difference from the horses. The ankle-joint also was of the same primitive character as in the other †Litopterna. The feet were relatively longer and more slender than in the other †proterotheres and the metapodial of the single functional digit longer in proportion to the phalanges.
Fig. 252.—Single-toed †proterothere (†Thoatherium minusculum), Santa Cruz. Restored by C. Knight from a skeleton in the museum of Princeton University.