F. B. Loomis — Wasatch and Wind River Rodents. 123 



Art. XII. — Wasatch and Wind River Rodents; by F. B. 



Loomis. 



Most of the rodents from the Wasatch and Wind River beds 

 have been loosely associated with the species of the Bridger 

 formation, largely, it would seem, in accordance to size. A 

 considerable number of specimens occurring in the Amherst col- 

 lection of 1904 from the Wasatch and Wind River beds show 

 at once that there was probably the same wealth of species in 

 these beds as in the Bridger beds, and further very casual 

 comparison developed that the species are characteristic in 

 each horizon. While the Amherst collection seems to contain 

 far more specimens from the lower beds than other collections, 

 for purposes of comparison the Bridger material of Yale 

 Museum was studied through and all the Tertiary rodent 

 material of the American Museum of Natural History was 

 generously loaned for generic and comparative study. It has 

 become clear that a revision of the Bridger species is desir- 

 able, but in this paper that material is used only for generic 

 characters. 



Two genera are involved, Paramys of Leidy and Sciuravus 

 of Marsh. The former has been well-defined and illustrated, 

 but of the latter, though the material is abundant, there is not 

 even a figure. Paramys was founded by Leidy in 1871.* 

 The genus has many sciurine features, being probably ances- 

 tral to this family. The skull is long and narrow, especially 

 so between the orbits, the width there being about \ the length. 

 The brain case is moderately swollen and has a trace of a 

 longitudinal crest along the dorsal median line. The ex- 

 ternal infraorbital foramen is not compressed as in squir- 

 rels, but broadly oval and opens almost directly above 

 premolar 3. On the lower jaw the broad area for the 

 attachment of the masseter muscle ends in front under 

 the interval between molar 2 and molar 3. Otherwise the 

 lower jaw is Sciurus-like. The dental formula is \ -g- f- § . 

 The molars are characterized by a strong protocone on 

 the inner side, and the hypocone either lacking or merely 

 indicated.f On the outer side the paracone and metacone are 

 strong cusps, with a small parastyle and tiny mesostyle also 

 developed. Intermediates (paraconule and metaconule) are 

 also present and usually more or less completely yoked to the 

 paracone and metacone. An extra marginal ridge is developed 

 front and rear. The lower molars are low crowned, the four 



*Proc. Acad. Nat. Sci. thila., p. 231. 



f For convenience Osborn's tritubercular terms are used withcrat commit- 

 ment to the theory. 



