528 University of California Publications in Geology [Vol. 9 



the fossettes ; and especially by the form of the protocone. In typical 

 Pliohippus the protocone is nearly circular in cross-section in the pre- 

 molars, but tends to flatten toward the posterior end of the molar 

 series. In the P. proversus type the protocone is relatively wide trans- 

 versely in the premolar region, but is flatter or narrower transversely 

 in all of the cheek-teeth than in P. pernix. 



The upper cheek-teeth in P. proversus and in the other members of 

 this group are distinguished from those of Equus especially by the 

 form of protocone (see figs. 13 to 16). In typical Equus this pillar 

 is normally longer anteroposterior^, flatter transversely, reaches 

 farther forward in advance of the isthmus connecting it with the pro- 

 toconule, and is concave rather than convex on the inner side. There 

 is also in Equus a stronger tendency to flattening of the outer faces of 

 the paracone and metacone, the mesostyle inclines to be thicker, and 

 the fossettes are usually narrower and have more strongly folded 

 walls. In E. stenonis of the Old World and in E. oecidentalis of Cali- 

 fornia, the distinctive characters separating Equus from the P. pro- 

 versus group are largely reduced in value. The form of protocone in 

 E. stenonis may approach that of typical Pliohippus in nearly all 

 characters, as it often tends to do in the milk dentition of modern 

 Equus. In E. oecidentalis the protocone is commonly short antero- 

 posteriorly and may be nearly as Pliohippus-like as in the known P. 

 proversus specimens. Both in E. stenonis and E. oecidentalis the com- 

 bination of all characters present in any given tooth is, however, com- 

 monly such as to indicate closer relation to modern Equus forms than 

 to the P. proversus group. 



The lower cheek-teeth of the upper Etchegoin P. proversus re- 

 semble those of a series described by Cope from the Blanco Pliocene of 

 Texas, and referred to P. simplicidens. The Blanco teeth (fig. 17) 

 are characterized by their considerable size, heavy cementation, and 

 by the anteroposterior length of the metaconid-metastylid column. 

 The metaconid-metastylid column commonly shows a rather sharply 

 angular internal groove. Cope 5 considered the character of this 

 column distinctive of P. simplicidens, in contrast with the Equus 

 forms of the Pleistocene. In P. simplicidens the outer walls of the 

 protoconid and hypoconid are convex to flattened. 



In the lower teeth of the P. proversus type from the upper Etche- 

 goin the crowns are large, long, and heavily cemented. The metaconid- 

 metastylid column in a number of specimens (figs. 5, 6, and 7) is wide 



s Cope, E. D., Geol. Surv. Texas, 4th Ann. Rep. for 1893, p. 66. 



