504 LAND MAMMALS IN THE WESTEEN HEMISPHERE 



In still another respect the feet of \Diadiaphorus deviated 

 markedly from those of the horses, viz. in the great proportion- 

 ate 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 capac- 

 ity, considering its geological date. The nasals were quite 

 short, though the degree of shortening was not such as to sug- 

 gest the existence of a proboscis. In general appearance the 

 skull recalls that of one of the larger foreodonts (p. 372) of 

 the North American Oligocene. 



To the genus \Proterotherium, the type of the family, be- 

 longed 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 jDiadiaphorus, 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 propor- 

 tions ; the nasal bones were considerably longer and the occiput 

 was somewhat wider. 



A more isolated position was held by the genus jThoa- 

 therium, 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 con- 

 nected, the two internal cusps and the anterior intermediate 

 cuspule being united into a nearly longitudinal ridge. The 



