532 University of California Publications in Geology [Vol. 9 



In considering the relation of P. proversus to forms of American 

 faunas later than the upper Etchegoin, it is interesting to note that 

 Equus occidentalis of the California Pleistocene represents the North 

 American species of Equus in which the characters of the cheek-teeth 

 show greatest simplicity. The gap between P. proversus and E. occi- 

 dentalis is too wide to permit the conclusion that the later of the two 

 is derived immediately from the earlier (see figs la to 10, 13, 16, and 

 18). In this connection it should be noted that the upper Etchegoin 

 stage, in which P. proversus occurs, is separated from the Pleistocene 

 horizon containing E. occidentalis by the Tulare formation, represent- 

 ing a thickness estimated by Arnold and Anderson' 1 to include at least 

 three thousand feet of strata. 



Of the Old World forms later than the upper Etchegoin stage of 

 the Pliocene, the type most nearly approaching P. proversus is Equus 

 stenonis," 1 described from the Pliocene and Pleistocene, and repre- 

 senting the least progressive Old World type of the genus Equus. In 

 some respects E. stenonis approaches the P. proversus type more 

 closely than does the American E. occidentalis. This may be due to 

 the origin of E. stenonis either in America or in Eurasia at a time 

 earlier than the Pleistocene stage of E. occidentalis. The Tulare 

 formation of California represents such a time in the geologic sequence. 

 The fauna of the Tulare is as yet very imperfectly known, and no 

 horse remains have been reported from these beds. It is probable that 

 a stage less advanced than E. occidentalis and more progressive than 

 P. proversus will yet be obtained from the Tulare section. 



The similarity in general characters of Pliohippus proversus of 

 the upper Etchegoin to the two Pliohippus species of the Blanco Plio- 

 cene, considered with similarity of the vertebrate faunas of the Blanco 

 and upper Etchegoin leaves little room for doubt that P. proversus. P. 

 simplicidens, and P. cumminsii represent closely allied forms of nearly 

 the same epoch. In the characters of both upper and lower cheek- 

 teeth the forms of this group are intermediate between typical Plio- 

 hippus and typical Equus, and represent the nearest approach to the 

 Equus type found among American Tertiary horses. The geologic 

 occurrence of these species represents a stage following the time of 

 maximum development of Pliohippus and apparently preceding the 

 earliest known occurrence of true Equus. The evidence presented in 

 this sequence of horse types in America indicates that some, at least, 



e Arnold, Ralph & Anderson, Robert, IT. S. Geol. Surv. Bull. 398, p. 147, 1910. 

 ~ See Major, Forsyth, C. J., Abhl. Sehw. Palae. Ges., vol. 4, Taf. 1 and 2, 

 1877; also ibid., vol. 7, Taf. 7, 1880. 



