MULUD.E— S1GM0D0NTES— II. LEUCOPUS EKEMIOUS. 



89 



nudis granulatis ; supra jlavido-grisea viz nigro limbata, lateribus jlavicantio- 

 ribus ; sublus alba, caudd obscure bicolore. 



Mouse about as large as H. leucopus, with rather longer and less hairy 

 tail and cars, naked palms and soles, of a pale yellowish-gray above, thinly 

 mixed with blackish hairs, more fulvous on the sides, the under parts white, 

 the tail obscurely tricolor. 



Habitat. — Valley of the Gila and Colorado. 



This highly interesting form of Hesperomys was based upon a few speci- 

 mens from the Colorado Desert, and none but the original ones appear to have 

 fallen under the notice of naturalists. In discussing its relationships to 

 leucopus, its peculiar habitat in the midst of the Great American Desert, in 

 the hottest region of the continent, must be kept prominently in view. Not- 

 withstanding the remarkable characters in some respects that it offers at first 

 sight, it will be found on closer examination to differ from leucopus solely in 

 characters readily superinduced by the isolated physical conditions under 

 which it lives. The ornithologist, in. particular, is fully prepared to meet 

 with the whole aspect of this case, from his knowledge of the modifications 

 in color that the birds of the Colorado Desert afford, notably in such 

 cases as those of Harporhynchus, Pipilo, &c. Nevertheless, the characters 

 accurately given by Baird mark all the specimens as at least a highly specialized 

 geographical race of leucopus. 



The palest specimens of Hesperomys that we have hitherto inspected 

 were from the comparatively dry and treeless regions of the Central Plateau ; 

 but these desert mice offer still more bleached coloration. The upper parts 

 are of a pale fulvous-gray, obscured along the middle of Ihe back by rather 

 few blackish hairs. The sides fade insensibly into a pah; brownish-yellow, or 

 dull tawny cinnamon (almost with a pinkish wash), which reaches down t lie 



