294 PROCEEDINGS OF THE NATIONAL MUSEUM vol.83 



distal end of a metapodial is relatively narrow and deep as compared 

 with the Idaho material, and a first and second phalanx in the collec- 

 tion are distinctly small, even for a hind foot. The first phalanx is 

 incomplete, but enough of it remains to show that it was much 

 shorter than in P. shoshonensis. 



P. proversus appears to be a somewhat earher stage of Plesippus 

 than found at Hagerman although this cannot be certainly demon- 

 strated without far more adequate material from the upper Etchegoin. 

 The material from the Tehama beds, referred by Vander Hoof ^^ 

 vvdth proper conservatism to P. proversus, exhibits a slightly more 

 complex pattern, but here again the Coalinga material does not 

 justify detailed comparisons. 



COMPARATIVE STUDY OF THE SKELETON OF PLESIPPUS 

 SHOSHONENSIS 



In the study and description of the skeletal structure of Plesippus 

 shoshonensis comparison is made primarily with the large zebra Equus 

 (Dolichohippus) grevyi. A large series of skulls of this species are 

 available in the National Museum collections, and being of a wUd 

 species they do not show the effects of domestication and selective 

 breeding so apparent in a series of Equus caballus skulls. Moreover, 

 the fossil species compares favorably in size of skull and limb propor- 

 tions with E. grevyi. The general skeletal material of E. grevyi in the 

 collection is, however, much more limited, and for this reason detailed 

 comparisons of vertebrae and limbs between fossU horse and the 

 recent zebra are not entirely satisfactory. The E. caballus vertebrae 

 and limb material are also scant. 



Skull. — The skull of Plesippus shoshonensis is elongate, with the 

 cranium relatively shorter than in E. grevyi. Perhaps the most 

 distinctive character of the fossil skulls is the greater angle between 

 the basicranial and basifacial axes than in modern species (pi. 24, 

 fig. 2; pi. 25, fig. 1; pi. 26, fig. 2). The degree of cytocephaly or 

 bending of the face on the cranium combined with the relative 

 shortness of the latter has given the basicranial area a noticeably 

 different appearance. The vomerine index is greater or the distance 

 from the palate to the posterior notch in the vomer is relatively much 

 greater with respect to the distance between the vomer and foramen 

 magnum (pi. 25, figs. 2, 3) than in either E. caballus or E. grevyi. 



The point of greatest deflection appears to be at the junction of the 

 vomer and the fused basisphenoid and presphenoid bones, with the 

 notch formed by the vomer and pterygoids more obtuse in the fossils. 

 The basioccipital and basisphenoid are broad and generally rounded 

 ventrally. In some old individuals of E. caballus the basioccipital is 



33 Russell, R. D., and Vander Hoof, V. L., Univ. California Publ. Bull. Dept. Qeol. Sci., vol. 20, no.2, 

 pp. 17-21, figs. 4-6, 1931. 



