Till-; MAMMALIA OK THE UINTA FOKMATIOX. 475 



tinet cusps. There is a single internal tubercle of the crown. In the thiid and 

 fourth molars of P. delicatissimus, I observe rudiments of a second internal tubercle. 



" The incisor teeth are compressed, with narrow anterior face. T!ie enamel is 

 not grooved and is little or not at all inllected on the inner side of the shaft, while 

 it is extensively so on the external face. There is a large vowndi foramen infnwrhitale 

 like that of Ischyromys and Fiber, and entirely unlike that of Gi/mno2)tychus and Sci- 

 unts, conforming in this respect to the forms of the extinct group of the Proloinyidui 

 of Pomel." 



PLESiARCTOiAn'S sciUROiDES S. & O. (PI. XT, Figs. 1-2). The specimen upon 

 which this species is founded consists of a skull, lower jaw and several liaib-bones. 

 It dilfers from the Wasatch and Bridger species principally in the character of the 

 superior molar teeth, in Avhich the transverse crests, wanting in the earlier members 

 of the genus, are clearly though not very prominently developed. There are also 

 several ditlerences of minor importance in the structure of the skull. These charac- 

 ters may possibly be regarded as of generic value, but the changes are so slight and 

 the agreement with the typical species of Plesiarctomys so close, that we do not feel 

 justified in forming a new genus. In size the Uinta species is very small, much in- 

 ferior to P. delicatissimus. 



The skull is very much like that of the last-named species, but with some not 

 unimportant differences. The muzzle is rather shorter, though both longer and 

 broader than in Sciurus ; the nasal bones are considerably broader behind than in 

 front, thus reversing the proportions found in Arctomys ; their anterior ends are some- 

 what thickened and slightly divergent, and posteriorly they terminate on the same 

 transverse line as the premaxillaries ; the latter are quite lai-ge and exhibit a consid- 

 erable surface upon the dorsal side of the skull. The frontals have a broad and nearly 

 straigbt anterior edge, but nari-ow very rapidly, showing a very strong postorbital 

 constriction of the ci-anium and no trace of postorbital processes, as is the very gen- 

 eral rule among the more ancient rodents ; they also seem to have a smaller antero- 

 posterior extent than in Sciurus, though this cannot be definitely decided. This poi*- 

 tion of the skull has quite a diffei-ent aspect from that seen in the two other species, 

 of which the structure of the cranium is known, P. delicatissimus and P. hians. It 

 differs from the former in that the postorbital constriction is less decided and less 

 elongated from before backwards, while the frontals are much more widely exi)anded 

 in advance of the constriction and over the orbits ; the superciliary ridges seem also 

 to be somewhat more prominent. So far as one can judge from Cope's ligure (J^o. 3, 

 PI. XXIV, Fig. 5) the postorbital constriction is deeper and the cranium more 

 tapering in front in /'. scinroides and P. hians. 



