328 University of California Publications in Geology [Vol. 12 



lower edge of the protocone of the premolars; (3) the comparatively 

 unplieated borders of the large and broad fossettes (the comparisons 

 made between teeth of equal and various stages of wear) ; (4) the much 

 greater transverse thickness of the lower dentition; (5) the relative 

 shallowness of the groove of the metaconid-metastylid column; (6) the 

 tremendous development of the fold between the protoconid and hypo- 

 eonid, and the much greater convexity of the outer faces of both the 

 protoconid and hypoconid. In short, the resume of the characters of 

 upper and lower teeth evidences the much greater primitiveness of 

 P. francescana as compared to E. bautistensis. 



The upper series of the new form is markedly heavier than the 

 teeth representing the types of Pliohippiis proversus Merriam and 

 P. simplicidens Cope. The character of the protocone is more primi- 

 tive than that seen in the P. proversus specimen. The P. simplicidens 

 tooth characters are discussed in detail below in a comparison with 

 P. cumminsii and the small San Timoteo horse. 



Important differences between the worn lower series referred to 

 the new form and those of the less worn series referred by Professor 

 Cope to P. simplicidens-^ are: (1) the much greater transverse 

 diameter; (2) the more rounded exterior faces of the protoconid and 

 hypoconid; (3) the broader fold between protoconid and hypoconid, 

 the production reaching the maximum within the metaconid-metastylid 

 column of the first molar instead of the second molar as in P. simplici- 

 dens; (4) the narrower and more produced parastyle; and (5) the 

 anteroposterior diameter of the first molar, which is the least of the 

 series, perhaps partly a condition of age. By all characters, as under- 

 stood, the new lower series represents a more primitive form than do 

 the lower teeth referred by Cope to P. simplicidens. 



Compared with certain small, Pliohippus-Uke lower molars referred 

 to the type P. proversus"'^ of the Upper Etchegoin the P. francescana 

 lower teeth are seen to be considerably larger, the anteroposterior 

 length of the metaconid-metastylid column to be greater, and the 

 exterior groove shallower. The teeth, however, show a similar degree 

 of primitiveness in the proportionate production of the fold between 

 protoconid and hypoconid and the convexity of the exterior faces of 

 the protoconid. 



25 Cope, E. D. Texas Geol. Surv., 1893; also Proc. Amer. Philos. Soc, vol. 30, 

 1892. 



26 Merriam John C. Eelationship of Equus to Pliohippus, Suggested by Char- 

 acters of a New Species from the Pliocene of California. Univ. Calif. Publ., 

 Bull. Dept. Geol., vol. 9, pp. 525-534, 1916. 



