332 



University of California Publications in Geology [Vol. 12 



Comparisons. — The small San Timoteo form is considerably more 

 primitive than the heavier type spe;eimens of Pliohvppus proversus, 

 differing markedly in: (1) the much lighter styles, as seen in the 

 corresponding third premolars; (2) the anteroposterior shortness, the 

 relative transverse thiclaiess, and the generally pronounced convexity 

 of the inferior border of its protocone; (3) the constriction of the 

 posterior protoconal fold, and the narrowness of its anterior end. 



The small upper teeth suggest a slightly more primitive form than 

 Pliohippus simplicidens or P. cumminsii (Cope) of the Texas Blanco. 

 The two former are unfortunately represented by very meager 

 material. P. cumminsii is based on the type molar and the fragment 

 of a second in the University of Texas Collection. P. simplicidens is 

 known by the tyi^e specimen in the latter collection, by an upper 

 tooth lacking the anterior portion of the protocone, by a referred 

 in the American Museum of Natural History, and by Professor Cope 's 

 referred lower series. A study of the type tooth of P. simplicidens, 

 as shown in Professor Henry F. Osborn's reconstruction, and com- 

 parison with the specimens from the San Timoteo would tend to indi- 

 cate through the following characters that the same is a P-, not a 

 molar as was originally stated: (1) the broad parastyle ; (2) the short, 

 thick, and round protocone, with slight concavity of the inner surface : 

 (3) the prominence of the accessory fold of the postprotoeonal valley; 

 and (4) the superior position occupied by the anterior horn of the 

 postfossette. IMoreover, these same characters might be those of the 

 P- of such a series as that represented by the two type molars of 

 P. cummitisii. The greater anteroposterior length of the P. simplici- 

 dens tooth, and the concavity of the inner edge of its protocone 

 are only the visual distinguishing characters between premolars and 

 molars. In short, a series thus compiled out of the P. simplicidens and 

 P. cumminsii types would be similar to that of P. franccscana minor 

 in: (1) size; (2) the anteroposterior length of the protocone; (3) the 

 simplicity of the fossette borders, and their strongly produced horns. 

 It would differ from P. francescana minor in the relative breadth and 

 thickness of its mesostyles, and in the broadness of the postprotoeonal 

 valley of the molar portion of the series. 



The referred and slightly worn upper premolar (folder 3, fig. 4) 

 bears a very interesting and remarkable resemblance to the type speci- 

 men of P. osiorni from the underlying Eden horizon, as discussed in 

 a following section. The small San Timoteo teeth in a less marked 

 manner suggest the genus Protoliippus, approximating the characters 



