KODENTIA. 179 



the Wasatch and Wind River, and two, P. delicatior and P. delicatissimus, to 

 all the Eocenes except the Washakie. 



Plesiarctomys bucuatus Cope. 



Report U. S. Geol. & Geog. Expl. Surv. W. 100th Meridian, G. M. Wheeler, iv, pt. ii, p. 171 ; pi. xliv , fig. 8. 



Plate xxiva, fig. 14. 



Jaws of four specimens which agree in proportions with those of this 



species found in New Mexico, were obtained by Mr. Wortman in the 



bad lands of the Big Horn basin, Wyoming. The species was established 



on a maxillary bone bearing teeth, while the present specimens are all 



mandibles. I do not detect any difference between these and the lower 



jaws of P. delicatissimus, excepting the inferior size of the former, from 



which, however, the first and last molars are wanting. I give the following 



measurements : 



Measurements. 



M. 



Length of inferior molar series 1 ~ 012 



Length of base of last molar 0038 



Diameter of inferior incisor 5 '^"tero-posterior 0033 



transverse 0008 



Depth of ramus at second molar 0088 



Plesiarctomys delicatissimus Leidy. 



Cope, Report U. S. Geog. & Geol. Expl. Snrv. W. of 100th Mer., G. M. Wheeler, iv, pt. ii, p. 172, pi. 

 xliv, figs. 9-12. Paramys deUcatissimus Leidy, Proceed. Acad. Phila. 1871, 231 ; Report U. S. 

 Geol. Sui-v. Terrs, ii, p. Ill, pi. vi, figs. 28-9. 



Plate xxiva, figs. 1-10. 



Poi'tions of the mandibles of four specimens of this rodent were 

 brought by Mr. Wortman from the Wind River Basin ; also, the skull of 

 another individual, and the greater part of the skeleton with the skull of a 

 sixth individual. 



The specimen last mentioned furnishes the following characters. The 

 skull has much the form of the large arboreal squirrels of the present day. 

 The muzzle is of moderate length, and the zygomata are not very widely 

 expanded. The skull is contracted just behind the eyes, for the orbits are 

 not defined posteriorly. Above the eyes the superciliary border is angu- 

 lar, but not prominent, and each one is continued as a delicate anterior tem- 

 poral ridge. The ridges converge backwards and unite into a low sagittal 

 crest opposite the anterior angle of the squamosal bone. The anterior supe- 



