438 PROCEEDINGS OF THE NATIONAL MUSEUM. 



ONYCHOMYS LETJCOGASTER MISSOTTRIENSIS (Audubon and Bachman). 



1851. Mus missouricnsis Audubon and Bachman, North Amer. Quad., vol. 2, p. 



327, pi. c. 

 1862. Mus missuriensis Wied, Verz. der auf seiner Beise Nord-Am. beob. Saug., p. 

 161 , in synonymy. 



Type-locality. — Fort Union, Montana. 



GeograjMc distribution. — Southeastern Alberta, southern Saskat- 

 chewan, northern and eastern Montana, western North Dakota, and 

 northeastern Wyoming. North to Calgary, Alberta, and Carlton, 

 Saskatchewan; east to GlenulUn, North Dakota; south up the Mis- 

 souri River to Bozeman, Montana, and up the branches of the Powder 

 and little Missouri Rivers into northeastern Wyoming. Cliiefly arid 

 Transition. 



General characters. — Size large, only sUghtly less than in true leuco- 

 gaster, with smaller hind foot. Coloration in all pelages much lighter 

 than in leucogaster, the immature especially paler than young of the 

 more eastern form. Darker, less buffy, than arcticeps. 



Color.— Adult in full winter pelage (189237, Dickinson, North 

 Dakota, February 14): Upperparts dark wood-brown, the head and 

 middle parts of body heavily darkened by the color of the darker 

 brown hair tips; underfur dark neutral-gray ; lanuginous ear tufts pure 

 white and very conspicuous, as are also the black and white mark- 

 ings on outer side of the ear. lips, lower cheeks, arms, hands, lower 

 legs, feet, lower sides, and entire underparts pure white, very sharply 

 marked from color of upperparts. Adult in early summer (168548, 

 Buford, North Dakota, May 8) : Like winter pelage but less bright in 

 color, the rich wood-browns faded and worn to drab and buff; the 

 head and back dark grayish-brown; and the white of underparts mixed 

 witli gray of the underfur. Juvenile (69045, Medicine Hat, Alberta, 

 October 13): Very much Ughter and gTayer, less blackish, than the 

 young of leucogaster. Upperparts mouse-gray, streaked with darker 

 hair tips; white ear tufts and black and white markings on the ears 

 conspicuous. Post-juvenile pelage (168546, Buford, North Dakota, 

 May 5): Like the juvenile but browner, less ashy mouse-gray; the 

 back and sides uniformly colored, without distinctly darker dorsum. 



Shull— The skull is shghtly smaller and less highly arched than 

 that of true leucogaster; the interorbital constriction is less, and the 

 braincase comparatively larger. 



Measurements. — Averages and extremes of eleven adults from 

 Alberta, Montana, North Dakota, and Wyoming: Total length, 150 

 (141-161); tail vertebriB, 39 (29-45); hind foot, 20.8 (20.0-21.5); ear 

 from notch in dry skin, 14.2 (12.6-15.8). Skull: Condylobasal length, 

 26.1 (25.4-26.6); zygomatic breadth, 15.3 (14.9-16.0); interorbital 

 breadth, 4.7 (4.5-4.9); breadth of braincase, 13.2 (12.9-13.6); length 

 of nasals, 10.9 (10.4-11.0); length of mandible, 15.6 (14.9-15.9); 



