THE MAMMALIA OF THE UIXTA FORMATION. 477 



run two crests, meeting at the internal cusp and thus forming a V. In Sciurus the crown 

 is somewhat expanded behind the posterior crest, thus making the inner side of the 

 tooth nearly as broad as the outer ; in 1*. sciuroides this expansion is much less 

 marked. M. 3 has but a single transverse crest with a broad basin-like expansion 

 behind it. The upper molars of Plesiarctomys are obviously of the same fundamental 

 type as those of Sciurus and Arctomys ; their chief diflerence from the latter consists in 

 the fact that the cingulum on the anterior side of the crown is not strongly developed 

 and is scarcely visible from the side, whereas in Arctomys, this portion of the cingu- 

 lum is very prominent and gives the side view of the tooth an appearance of being 

 composed of three diistinct transverse crests. 



The lower teeth are much worn, and it is therefore rather difficult to analyze 

 their construction. The single premolar is composed of a pair of anterior cusps, of 

 which the inner one is high and pointed, the outer one very small and but faintly 

 distinguished from its neighbor, and a low heel. The molars have four cusps sur- 

 rounding a median depression, of which the antero-internal one is much the most 

 prominent. The presence or absence of connecting crests could not be clearly made 

 out. The resemblance of these teeth to the lower molars of Sciurus is very close. 



The humerus is short and very slender with large head and low, inconspicuous 

 tuberosities ; the bicipital groove is distinctly deeper than in /*. delicatissimus, and 

 the deltoid ridge is quite long and prominent. The distal end of the specimen is 

 broken, but enough remains to show that the epicondylar foramen is present and that 

 the supinator ridge is somewhat less prominent than in the Bridger species so fre- 

 quently referred. Whether the Uinta species had such a prominent internal cpicon- 

 dyle is not certain. 



The tibia is quite long, slender, and strongly arched forwards ; the distal end is 

 quite deeply grooved for the asti-agalus and the internal malleolus is well developed. 

 The fibula, although not preserved in the specimen, was evidently entirely distinct, as 

 the outer face of the distal end of the tibia is uninjured and exhibits no trace of co- 

 ossification with the fibula. 



Aleat'irements. M. 



Length of skull OKI 



Brcadlh of skull in front of orbits 025 



" behind ■' 010 



Length of lower jaw 028 



Lcnglli upper molar scries 013 



lower " " Oli 



Anteroposterior diameter of lower incisor OOH 



Transverse " " •• W2 



Length of humerus (esliniiilid) 04ii 



•' of libia 0«0 



A. V. S. — VOL. XVI. 'h. 



