MAMMALS OF THE MEXICAN BOUNDARY. 



375 



FlG.63.— Onychomystoekidus. a.FORE- 

 foot; 6, Hindfoot; c, Tail (nat. size) . 



GeograpUcal range.— SonoTan Zone of the Elevated Central Tract. 

 Description. — Size and proportions intermediate between those 

 of Onychomys torridus arenicola and 0. t. perpallidus. Pelage dense 

 and beautifully soft and lustrous. Color above cinnamon, slightly 

 grayish, considerably darkened above by black annuU and tips to 

 the hairs, and inclining to ochraceous 

 on the sides. Ears with , lanuginous 

 white tufts at their anterior base; well 

 clothed with whitish hair with a black 

 spot on the anterior half of their con- 

 vex surfaces. Feet (fig. 63a and I), 

 under parts, and end of tail white. 

 Tail (fig. 63 c) densely hairy, with a 

 dark line of drab, mixed with hoary 

 tips to the hairs, on upper side of basal 

 two-thirds. Whiskers long, reaching 

 past the shoulder; their color, mixed 

 black and white. The measurements 

 are shown in the accompanying table. 

 There are but three individuals in 

 the series of twenty-two specimens 

 before me which are in the gray pelage of the young. Two of 

 these, about two-thirds grown, taken May 29 and September 18, are 

 gray above and white below, with the usual black spot on the ante- 

 rior band of the ear. Both are in summer pelage, with the gray 

 underfur showing between thfe hairs of the sparse coating of the 

 under surface. The third specimen, taken November 

 23, is in winter pelage and somewhat older, with the 

 sides tinted with drab. The pelage is white below and 

 very dense. The shade of gray is darker in young 

 specimens of the subspecies torridus than in arenicola 

 or perpallidus, but considerably paler than in the sub- 

 species ramona, from the Pacific coast. 



Seasonal variation of adults. — There is very little 

 seasonal difference in coloring. Specimens taken in 

 April, May, and June are colored almost exactly like 

 those captured during November and December. 

 Perhaps the latter are a trifle more grayish or silvery, 

 with the fur more dense and glossy. 



Granial and dental characters. — ^The skull, except for its smaller 

 size, closely resembles that of Onychomys rrbelarwphrys. It nieasures 

 25.7 by 13.6 mm. (For teeth see fig. 64.) 



Bemarlcs. — In coloration the subspecies torridus agrees very closely 

 with 0. melanophrys, which has a similar geographical distribution. 

 Similarly, 0. torridus arenicola is the analogue of 0. pallescens. We 



Fig. 64.— Onycho- 

 mys TORRIDUS, 



a, Lower mo- 

 lars; 6, Upper 



MOLARS. 



