376 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [FoZ.VI. 



MYOMORPHA. 

 EIBITS (Liedy nom.) Cope. 



A.rmiial Report of the U. S. Geological Survey of the Territories, F. V. Hayden 

 in charge, 1873 (1874), p. 474. — Eumys Leidy (name only). Proceedings Academy 

 Philada. 1856, p. 90; loc. cit. 1857, p. 89; Extinct Mamm. Dakota and Nebraska, 

 p. 342. 



Dental formula: I. |; C. §; M. |. Crowns of the superior molars sup- 

 porting two external cones, and two internal tubercles of crescentio 

 section which communicate with the former by transverse ridges. In- 

 ferior molars of similar constitution, but reversed, the conic tubercles 

 being interior and the crescentic exterior. The posterior tubercles of the 

 posterior molars reduced, and an additional one on the anterior extremity 

 of the first molar. Superciliary ridges none, but the supraorbital bor- 

 ders converging towards the middle line, and meeting above the post- 

 orbital region. iN'o indication of postfrontal processes. Infraorbital 

 foramen rather large above, terminating below in a vertical fissure. Inci- 

 sive foramen entering the maxillery bone extensively. Incisor teeth not 

 grooved. 



I only Tsnow this genus from the cranium anterior to the pterygoid 

 region, the mandibles, and the dentition. These parts display the char- 

 acters of the 3Iuridce, and in particular of the existing genus Hesperomys. 

 The only character which I can find which has enabled me to distinguish 

 Eumys from the latter genus is the extension upwards of the orbital fos- 

 sne so as to form an interorbital crest. In none of the Sigmodont genera 

 of North America are the supraorbital borders contracted in this way, 

 but the crest is seen in Fiber and in various degrees in the genus Arvi- 

 cola, being as distinct as in Eumys in A. xantJiognathus.* 



A single species is certainly referrible to this genus, the E. elegans, 

 which was abundant during the White Eiver Miocene epoch. I have 

 referred to the same genus a second species, in which the same charac- 

 ters are seen in the inferior molars; but as the frontal region is un- 

 known, the reference was provisional only. This is the Hesperomys 

 loxodon* Cope, of the Loup Fork Miocene of New Mexico, a much smaller 

 species than the E. elegans. 



The typical species was originally described by Leidy, who gave it 

 the generic name which I have adopted ; but he at no time character- 

 ized the genus, or showed how it differed from others already known. 

 This was first done by myself as above cited. 



HESPEEOMYS Waterhouse. 



This recent genus had a representative in the Miocene period in North 

 America, so far as the characters of the skull and dentition may be con- 



* See Coues, U. S. Geol. Surv. Terrs. XI, PL IV, fig. 54. 



