EODENTIA. 175 



M. 



Length of diastema 0060 



T^ .„ . . . < anteroposterior 002(! 



Diameters incisor < '■ 



< transverse 0020 



Depth of ramus at diastema 0070 



Depth of ramus at middle of P-m. iv 0000 



The length of the skull of this animal was about equal to that of the 

 Norway rat Mus decimianus, but the large proportions of the fourth premolar 

 indicate that the cranium was much deeper than in that animal, and had 

 probably the form of that of the Thylacoleo carnifex* 



RODENTIA. 



Remains of species of this order are not abundant in the beds of the 

 Wasatch epoch, and are rather common in those of the Wind River and 

 Bridger. They are not very i^arious as to type, and the greater number are 

 apparently allied to the squirrels. 



PLESIARCTOMYS Bravard. 



Plesiaretomys Bravard, Ossemens fossiles de Desbruge, 18r)0, p. 5. — Gervais, Pal^ontologie franjaise 



exijlic, tab. 30, p. 4. 

 Paramys Leidy, Report U. S. Geol. Surv., 4to, i, 1873, p. 109; Proc. Pbila. Acad., 1870 (name only). 

 Pseudoiomus Cope, Paleontological Bulletin No. 2, p. 2, August 3, 1872, nomen nudum; Annual Report 



U. S. Geol. Surv. Terrs., 1872 (1873), p. 610 (defined). 



The inferior molars by which this genus has been generally known 

 resemble much those of existing Sciuridce, but there are cranial characters 

 which distinguish it from the existing forms of that family. 



The crowns of the inferior molars support four rather small and strictly 

 marginal tubercles, which inclose a median valley. The anterior inner 

 tubercle is more elevated than the others, and the posterior -two tubercles are 

 connected by a low ridge on the posterior border, which may be more or less 

 tubercular on the last molar. In some of the species, the marginal tubercles 

 are merely elevations of the margin, while, in others, the adjacent tubercles 

 of a pair approximate, so as to form a pair of interrupted cross crests. 



There are five superior molars, of which the anterior is of small size. 

 They resemble tliose of Sciurus, but the transverse crests are obsolete or 



*A restoration of the skull of this animal is given by Flower, Quarterly Jonrn. Geolog. Society, 

 vol. xxiv, 1868. 



