1885.] PROCEEDINGS OF tJNlTEl) STATES NATIONAL MUSEUM. 407 



All of the specimens of Hesperomys loaned me for comparison by the 

 National Museum have the upper parts of a bright yellowish-brown, 

 inclining to a tan color, while the parts beneath are of a pure white, 

 the line of demarcation between the two being very distinct. Now, 

 all the specimens of Hesperomys from this region are of a very pale, dull 

 clay color above, and white beneath, but the line of demarcation not so 

 evident at first sight, in consequence. 



The dental formula in True's mouse agrees with the Murincc generally, 

 being, incisors | and molars |-| ; we notice, however, in the specimen 

 that the cusps of both the mandibular and upper molars have been en- 

 tirely worn off, leaving a shallow, longitudinal groove along their upper 

 surfaces, connecting the three crowns. I am unable to say at the present 

 writing what this is due to, or how constant it may be. To decide this 

 we must have a better knowledge of the character of the food of this 

 mouse, as well as the opportunity to examine specimens of different ages. 



R. leucopus of this region, inhabiting as it does the valleys, no doubt 

 subsists upon the softer vegetable substances which give rise to but 

 little abrasion of the molar teeth, whereas H. Truei, a species which 

 has chosen the rocky mountain sides for its home, now lives upon the 

 flinty piiion nuts, and the spiny leaves of the trees upon which they 

 grow, and these substances may have something to do with the wear- 

 ing down of the molar teeth. The implantation of their roots agrees in 

 general with that observed in other members of the genus, and these 

 nou- visible parts are equally well developed in them. 



In the mandible of Hesperomys, we find the condyle and the small 

 coronoid process separated by a long, shallow, longitudinal valley 

 (Figs. 7 and 8, pi. XXI). 



Again, in Hesperomys the nasal processes of the maxillaries do not 

 project behind the nasals, as they invariably do in Neotoma (Figs. 4 

 and 5, pi. XXI, N. Mx., N. Mx'.). It is of interest to know also in re- 

 gard to these two genera, that the ratios of the longitudinal and trans- 

 verse diameters of the interparietal bones differ very markedly, 



Hesperomys Truei, sp. nov. 

 True's Fifion Mouse. 



DESCRIPTION. 



(Adult male, in the flesh.) 



A Hesperomys with a form more robust than in most others of the 

 genus. Eyes large and capable of extraordinary protrusion when the 

 animal is affrighted. Whiskers very long and many-haired. Ears pre- 

 senting the normal generic characters, but remarkable for their unusual 

 size. Upper halves are delicate, thin, and very sparsely covered with 

 shoi t, fine hairs. Fur rather longer than common, soft, loose, and flossy. 

 Tail niore thickly haired than in H. leucopus, and the hairs longer. 



Color. — Upper parts of a yellowish brown with an admixture of black- 



