292 PROCEEDINGS OF THE NATIONAL MUSEUM vol.83 



these characters can be observed. In Hagerman specimens of equal 

 vertical length the pattern shows them to be less worn, as the enamel 

 lakes are not then isolated as they are in the Payette tooth. More- 

 over, the lakes in the latter specimen are more constricted with the 

 margins somewhat more elaborately crenulated than is usual in 

 P. shoshonensis. 



The foregoing observations would appear unimportant were it not 

 for the large number of dentitions available for comparisons, which 

 give reasonable definition to the variability of P. shoshonensis. The 

 greater size of Equus idahoensis occurring at more western localities 

 in the Idaho beds is further suggested by foot material obtained in 

 1934 from deposits exposed along Sinker Creek, However, a last 

 upper molar (U.S.N.M. .no. 687), collected by Clarence King on 

 Sinker Creek and considered by Leidy to represent Equus excelsv.s, 

 can be closely matched in the Hagerman collection. 



The genotype, Plesippus sim.plicidens (Cope), is based on a single 

 upper cheek tooth and referred lower teeth from the Blanco beds of 

 Texas but more adequately represented by skeletal material in the 

 American Museum of Natural History collected by Matthew and 

 Simpson. This material formed the basis of Matthew's ^^ description 

 of Plesippus. 



The skull of Plesippus simplicidens (Amer. Mus. no. 20077) is 

 within the size range exhibited by skulls of P. shoshonensis. The 

 cranium is relatively a little longer and the facial length appears 

 relatively less, even though the premaxillae are restored with a 

 greater diastema between I^ and P^ The laclirymal fossa is notice- 

 ably developed, as in Idaho specimens where this character is not 

 obscured by crushing ; moreover, a very shallow malar fossa is present. 

 The nasals are slightly concave longitudinally and narrow trans- 

 versely over the laclirymal fossae and are also infolded along the 

 median line. The supraoccipital is narrow and somewhat over- 

 hanging though not so conspicuously so as in the Idaho form. The 

 basicranial line is deflected from the basifacial line to a marked 

 degree as in P. shoshonensis, but the angulation is not so acute 

 at the contact of the basisphenoid and vomer, the longitudinal 

 profile of the ventral margin of the basioccipital and basisphenoid 

 being more concave. The ventral surface of the basioccipital and 

 basisphenoid is rounded transversely and is slightly rough but not 

 keeled as observed in some Equus caballus skulls from individuals of 

 equally advanced age. The external audital tube, as in P. sho- 

 shonensis, is directed upward and backward to a marked degree. 

 Also, as in the Idaho skulls, the distance from the palate to the 

 posterior margin of the vomer is distinctly greater than the distance 

 from the vomer to the foramen magnum. 



«i Matthew, VV. D., Amer. Mus. Nov., no. 131, p. 2, 1924. 



