FOSSIL VERTEBRATES FROM SAN PEDRO VALLEY—GAZIN 495 
ture, especially toward the ends of the series. In the lower dentition 
the curvature is almost entirely in an anteroposterior plane. The 
cheek teeth exhibit moderately complex folding of the enamel and 
the isolated protocone in the upper teeth is generally small and 
smoothly oval in outline in moderate to advanced wear, although in 
early stages of wear the protocone may be rather irregular in outline. 
The upper tooth here figured (fig. 40), probably M?, measures about 
17.5 by 15.6 mm. in cross section and is about 65 mm. long. 
The feet are 3-toed, as in Hipparion, but strikingly small and slen- 
der. The first phalanx of a third digit measures 51 mm. long, and 
23.5 and 20.0 mm. wide at the proximal and distal ends respectively. 
The hoof is scarcely more than 30 or 35 mm. wide. 
PLESIPPUS sp. 
In his preliminary report, 1922, on the San Pedro Valley faunas 
Gidley noted the presence in the Benson collection of horse material 
other than Hipparion, which he referred to Pliohippus. Later, how- 
ever, he regarded these as representing Plesippus, as indicated by 
notations on the specimen labels. The material of Plesippus is rather 
scant and consists of a few isolated cheek teeth, most of which are 
incomplete, and a few toe bones. 
The teeth are large and robust, being generally heavier than in the 
modern horse, but exhibit certain somewhat more primitive char- 
acters distinguishing them from Quaternary forms, particularly in 
the sharpness of the reentrant between the metaconid and metastylid 
columns of the lower teeth. The teeth and portions of teeth show a 
striking resemblance to those in Plesippus shoshonensis from Hager- 
man, Idaho, being in a nearly equivalent stage of development. The 
toe bones can be closely matched in material of P. shoshonensis. As 
in the zebra, these are relatively small and slender as compared with 
those in EHquus caballus. 
Order ARTIODACTYLA 
PLATYGONUS sp. 
Several isolated teeth and a maxillary fragment with two decidu- 
ous premolars belong to a species of Platygonus. The teeth are of 
moderate size, close in this respect to those in Platygonus pearcei™ 
from the upper Pliocene at Hagerman, Idaho. They are more rugose 
than in P. pearcei, the cingulum better developed, and the an- 
terior and posterior crests seem more sharply separated. 
8 ¢, L, Gazin, Journ. Washington Acad. Sci., vol. 28, no. 2, pp, 41-49, figs. 1-3, 1938. 
