436 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.47. 



GeograpJiic distrihution.— Southern Manitoba, eastern North Da- 

 kota, northeastern South Dakota, and parts of extreme western 

 Minnesota, west to Minot and Fort Clark, North Dakota, Transition 

 zone. 



General characters. — Size large, slightly greater than in the neigh- 

 boring forms, missouriensis and arcticeps; coloration much darker, 

 the young especially blackish. Skull large; in adult with braincase 

 shortened anteriorl}^ and lengthened, more overhanging, posteriorly. 



Color. — Adult in fresh pelage (55462, Devils Lake, North Dakota, 

 August 5): Upperparts dark drab-brown, finely lined with darker; 

 top of head and broad area along center of back especially dark, 

 more lined by dark browTi hair tips; nose grayish-drab; underfur 

 of upperparts broadly blackish-slate; lanuginous tufts at bases of 

 ears pure white, very conspicuous; outer sides of ears blackish-brown 

 on front half, whitish posteriorly; lining of ears white; tail drab- 

 brown for two- thirds its length above, the terminal third and under 

 side whitish. Lower sides, lower cheeks, and entire underparts 

 white, sharply marked from color of upper sides and back, the hairs 

 of chin, throat, and arms white to bases; underfur of middle chest 

 and belly narrowly blackish-slate; arms and hands, inner sides of 

 legs, and feet white; outer sides of legs with narrow stripe of color 

 like back extending to near foot. Adult in worn and faded coat, 

 before renewal (180025, Hankinson, North Dakota, July 23): Like 

 fresh coat but much shorter haired and with less distinct dark area 

 along back. Entire upperparts uniform dark drab-brown, with 

 little streaking from hair tips, the darker tips of longer haii-s having 

 disappeared; ear tufts virtually wanting, of same color as head and 

 not noticeable; markings of ears, legs, and tail less conspicuous than 

 in fresh pelage. Underparts gra}nsh-white, the underfur showing 

 through the wliite of thinly haired breast and belly. Adult, mel- 

 anistic phase (180024, Hankinson, North Dakota, July 23): Entire 

 upperparts glossy brownish-black, the hair tips only slightly bro\\Tier 

 than the underfur. Underparts shghtly paler, vnih blotches of pure 

 wliite on throat, breast, and arms. Juvenile (189252, Traverse, 

 South Dakota, July 4): Nose, head, and center of back to tail dark 

 blackish-brown; sides slightly paler, more grayish; no hghter colored 

 ear tufts; black markings of forward part of outer ear conspicuous. 

 L^nderparts white, mixed with grayish from the underfur. Post- 

 juvenile pelage (202088, Fort Clark, North Dakota, July 24): More 

 slaty-black, less brownish, than the first pelage. Upperparts dark 

 grayish-drab, sides lighter; ear tufts whitish, conspicuous; under 

 parts very sharply marked from color of sides, whitish with slate- 

 gray underfur on chest and belly. Much darker than the young of 

 missouriensis or arcticeps. 



Slcvll. — The skull of Onychom,ys leucogaster leucogaster is larger than 

 that of any other form of Onychomys. It slightly exceeds in general 



