290 PROCEEDINGS OF THE NATIONAL MUSEUM vol.83 



This form is of the size and limb-proportions of the average Pleistocene species 

 of Equus, much larger and more robust than true Pliohippus. The teeth are 

 most like Pliohippus, but longer crowned and less curved, with heavier meso- 

 etyle and larger more nearly isolated protocone. The cement lakes of the upper 

 molars are narrower, and their enamel borders have a few simple inflections. 

 The skull has the elongate proportions of Equus, in contrast to the shorter 

 skull of typical Pliohippus and earlier equids. It retains in the forefoot tiny 

 vestigial nodules representing the trapezium and fifth digit, progressively reduced 

 in the earlier stages of Equidae, normally absent in modern Equus. The splints 

 are from two-thirds to three-quarters of the length of the cannon bone, nearly or 

 quite as much reduced as in Equus, while in true Pliohippus the splints are almost 

 as long as the cannon bone, and it is not certain that the lateral phalanges had 

 entirely disappeared. 



After further study of the Blanco material Matthew ^^ observed 

 that "the animal compares in size and most proportions with the 

 Arab horse, but the hoofs are much smaller, the back shorter, and the 

 barrel less rounded and convex." He also noted that the lateral 

 digits or splints end "in a thin flattened and expanded sliver instead 

 of the little knob of Equus", and that "the fibula is reduced to a 

 splint which in Plesippus is distinctly shorter than in any species of 

 Equus." Moreover, the skull was observed to have shallow facial 

 pockets or fossae, more reduced than in Pliohippus. 



A number of outstanding characters are noted in the Hagerman 

 material, many of wliich were observed by Gidley in the more limited 

 collection he had at his disposal, and most of these are probably of 

 generic significance. The cranium is short and markedly deflected 

 from the distinctly elongate facial portion. The preorbital fossa 

 anterior to the laclu-ymal bone is conspicuously developed along the 

 line of the suture between the nasal and maxilla. The rostrum is deep 

 anteriorly and narrow across the nasals. The first premolar above is 

 well developed and almost always present. The first premolar below 

 is simple and commonly present in young individuals. The cheek 

 teeth approach in appearance those in Equus, somewhat more so in the 

 earlier stages of wear. The feet are comparatively small, and the 

 splint bones are reduced but with their average length relatively 

 greater than in Equus. A few trapezia were found, but very nearly 

 all the trapezoids show well-marked facets as evidence for articulation 

 with a trapezium. A single fifth metacarpal was discovered. These 

 apparently left little or no impression on the fourth metacarpal or 

 outer splint as evidence of their possibly more general occurrence. 



PLESIPPUS SHOSHONENSIS Gidley 



Specific characters. — The characters of the species, though not 

 clearly separated from those defining the genus, may be briefly out- 

 lined as follows: The skull is large and the relatively short cranium 

 exhibits a narrow, sharply overhanging occipital crest. The teeth 



" Matthew, W. D., Quart. Rev. Biol., vol. 1, no. 2, pp. 162-164, 1926. 



