440 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.47. 



over much of back, rump, and hips; paler on face, shoulders, and 

 back of ears; ear tufts white but rather inconspicuous against the 

 general light coloration of head; markings of outer side of ears much 

 subdued, brown on forward half, whitish, scantily haired posteriorly. 

 Ai-ms and hands, lower legs and feet, and entke underparts, pure 

 white, the underfur of lower breast, sides, and belly faintly darkened 

 with pale gray. Tail wliite with narrow stripe of brownish above 

 for two-thhds its length. Adult in short summer coat (54452, Colby, 

 Kansas, June 28; breeding female): Entu'e upperparts light drab, 

 finely lined by the darker brownish hau- tips; underparts grayish- 

 white, the gi-ay underfur showing tlirough the white of hair tips on 

 the scantily furred breast and belly. No lanugmous ear tufts. 

 Adult in fall coat (150985, Gaume's Ranch, Baca County, Colorado, 

 November 28): Upperparts avellaneous, everywhere finely streaked 

 and darkened by the blacldsh hair tips ; top of head and middle of 

 back and rump slightly darker than face and sides; ear tufts buffy; 

 underparts white. Juvenile (54455, Colby, Kansas, June 28) : Above 

 uniform light mouse-gi-ay, the hau's pale gray at bases, darker gray 

 in middle, and lightly tipped with pale brownish-gi-ay. Underparts 

 grayish-white; no ear tufts. Young adult, or post juvenile, in early 

 winter (8529, Philadelpliia Acad. Sciences, type; Clapham, New 

 Mexico, November 7) : Like adult in summer but darker and richer 

 colored, more brownish; the long full hahs of head and back with 

 broad subterminal bands of dark buff and tips of blackish; lower 

 sides brownish; underparts whitish; the underfur dark neutral-gray. 



Skull. — Alm ost precisely as in missouriensis. 



Measurements. — Averages and extremes of 35 adults from South 

 Dakota, Nebraska, Kansas, Colorado, New Mexico, and Texas. 

 Total length, 151 (131-170); tail vertebrae, 42 (34-53); hind foot, 

 20.9 (18.5-23.0) ; ear from notch in dry skin, 13.4 (12.4-14.9). Skull: 

 Condylobasal length, 26.4 (24.5-27.7); zygomatic breadth, 15.4 (14.4- 

 16.6); interorbital breadth, 4.6 (4.2-5.0); breadth of braincase, 13.0 

 (12.4-13.7); length of nasals, 11.2 (10.1-12.2); length of mandible, 

 15.6 (14.6-16.8); maxillary tooth row, 4.6 (4.3-4.8). For detailed 

 measurements of specimens see page 474. 



Type-specimen. — No. 8529, Academy of Natural Sciences, Phila- 

 delphia. Sldn and skull of young male (teeth unworn and basal 

 suture open) in fresh fall pelage of the immature. Collected Novem- 

 ber 7, 1893, by Ernest E. Thompson. Rhoads Collection, No. 1529. 



Remarks. — Specimens of 0. I. arcticeps from South Dakota and 

 northern Nebraska are absolutely indistinguishable from specimens 

 collected on the Staked Plains in New Mexico and Texas. Intergra- 

 dation with neighboring forms takes place wherever the ranges meet 

 and it is sometimes difficult to identify immature or poorly furred 

 specimens with any particular form. Skins from Santa Rosa, New 



