430 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.47. 



Bangs, Museum of Comparative Zoology at Harvard; and to Dr. 

 Witmer Stone, Philadelphia Academy of Sciences, I am indebted for 

 the loan of valuable material, all of which was assembled by the 

 Biological Survey, through the courtesy of Mr. E. W. Nelson. In 

 the lists of specimens examined I have indicated the source of all 

 material from localities not represented in the United States National 

 Museum collections. Mr. Vernon Bailey, of the Biological Survey, 

 has given me much information regarding physiographical conditions 

 in localities from which the small number of specimens examined 

 hardly warranted the mapping of boundaries of forms, without inti- 

 mate knowledge of the country. 



SYSTEMATIC DISCUSSION. 



Genus ONYCHOMYS Baird. 



1857. Onychoviys Baird, Gen. Rep. North Amer. Mamm., p. xxviii and p. 457 



(subgenus of Hespcromys Waterhouse) . 

 1874. Onichomys Giglioli, Boll. Soc. Geogr. Italiana, vol. 11, p. 326. 



Type-species. — Hypudaeus leucogaster Wied. 



Diagnosis} — Form stout, tail comparatively short, thick, and 

 tapering to an obtuse point, the end usually white. Fore feet larger 

 than in Peromyscus, five-tuberculate. Hind feet with only four 

 tubercles, all phalangeal; the sole densely furred from heel to tuber- 

 cles. Mamm8e,pectoral, 1-1; inguinal, 2-2 = 6. Nasals wedge-shaped, 

 terminating posteriorly considerably behind the end of the nasal 

 branch of the premaxillaries. Coronoid process of mandible well 

 developed, rising high above the condylar ramus and directed back- 

 ward in the form of a large hook. First and second upper molars 

 large; third less than half the size of the second. First upper molar 

 with two internal and three external cusps, the anterior cusp when 

 unworn unequally divided at summit into two or three cusplets, 

 narrow, and on a line with the outer cusp row, leaving a distinct 

 step on the inside. Second upper molar with two internal and two 

 external cusps, and a narrow antero-external fold. Last upper 

 molar broader than long or subcircular in outline. First lower molar 

 with an anterior, two internal and two external cusps, and a postero- 

 internal loop. Second lower molar with two internal and two 

 external cusps, an antero-external and a postero-internal fold. Third 

 lower molar scarcely longer than broad, subcircular in outline. 



Color pattern. — Sharply bicolor; head, back, and upper sides col- 

 ored; underparts white, with line of demarcation sharply drawn; 

 lanuginous tufts at bases of ears commonly whitish or buffy and con- 

 trasting with general color of head and ears ; arms, hands, inner sides 

 of legs, and feet whitish; tail usually bicolor, with tip whitish. 



1 With slight modification, taken from Merriam, North Amer. Fauna, No. 2, pp. 3-1. October, 1889. 



