476 THE MAMMALIA OF THE UINTA FORMATION. 



As in the other species the infraorbital foramen is in the species before 

 us large and situated more immediately beneath the orbit than in Sciurus. The 

 shape of the lachrymal cannot ver}'^ well be made out. The parietals are apparently 

 very long ; they are broader than in Arctomys, narrower than in Sciurus, and the sagit- 

 tal crest is barely indicated. The squamosal has, of course, a corresponding de- 

 velopment ; the zygomatic arches are very slender and compressed ; they do not 

 arch out very strong!}' from tlie sides of the cranium and are somewhat shorter than 

 in P. delicaUssimus. The anterior part of the arch is shaped much as in Sciurus, but 

 the jugal has a somewhat greater vertical depth. 



The occiput is very broad and low ; in the median line it forms a slight convexity 

 over the vermis of the cerebellum, and on each side of this there is a rather deep 

 depression. The paroccipital processes are very short, and the foramen viagmim 

 veiy large. The base of the skull is much injured, but enough is preserved to show 

 that the basi-occipital is rather broad and flat, and that the tympanic bullae are mode- 

 rately inflated. The posterior nares are rather long and their anterior edge extends 

 to the front of the last molar ; in Sciurus and Arctomys it is altogether behind the 

 molar series. The mandible is relatively more robust than m P . delicaUssimus ; the 

 masseteric fossa is, however, not so deep, and extends forward to beneath the second 

 molar ; the coronoid process is shorter, more slender and pointed, and the notch be- 

 tween it and the condyle is not so deep. 



The train is in general like that of P. Mans, but the cerebellum is proportion- 

 ately larger, the hemispheres shorter, broader behind and tapering much more rapidly 

 forwards.; the olfactory lobes are also narrower. 



The Dentition. The molar formula of this species, like that of the other mem- 

 bers of the genus is pra. ?, m. |. In the upper series the penultimate premolar (pm. 



3) is implanted by a single fang and has a very small and sim- 

 ple ci-own, thus dittering strongly from the large pm. 3 of 

 Arctomys. Pm. 4 is nearly as large as the molars. The 

 crown is of triangular shape and consists of three tubercles at 

 /£x:J^ the anales of the crown, with a fourth one intercalated be- 

 DIAc..l.-UppeI•andlo^yermo. tween the internal and postero- external cusps, thus forming 

 \iu-& oi p. sHuroides. xf an interrupted posterior crest; the antero-external cusp is the 

 largest and most prominent of the three. This tooth resembles much the correspond- 

 ing one of Arctomys, but with the difference that the posterior line of tubercles have 

 not coalesced to form a crest and that the cingulum is much less strongly developed 

 on the front of the crown. The molars are very closely like those of Sciurus, but of 

 more distinctly triangular shape. In m. 1 and 2 there are two external cusps from which 



