MURIDAI—SIGMODONTES—H. LEUCOPUS EREMICUS. 89 
nudis granulatis ; supra flavido-grisea vix nigro limbata, lateribus flavicantio- 
ribus ; subtus alba, caudd obscure bicolore. 
Mouse about as large as H. leucopus, with rather longer and less hairy 
tail and ears, 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 bicolor. 
Hapirat.—Valley of the Gila and Colorado. 
TABLE XXIV.—Measurements of six specimens of HrsPEROMYS LEUCOPUS EREMICUS. 
Nose to— £ 
Pt Sc Sa ae | La p=] ~~ a Nr j- 
i) Locality. Collector. = | 5 2 & RIGAEOCE TE 
2 a : - Se eS men. 
EI 3 SMP Se latee Wee zg z BE 
¥ ad 3 3) i Ci ° = S 
A 8 = o & oH & iss) loot 
1581 | Fort Yuma, Cal .........-. Ge homass=—o-| =~ 5) e==-- 1.08 | 3.17 | 4.08 |...... | 0.76 |...-.. Dry. 
2074 |.....9 (se saccasccscencas peasy oes 0.95 | 1.07 | 2.77 | 3.70 | 0.30 | 0.80 | 0.60 | Alcoholié. 
PST pases (i tecaoSeSaecp aes re atil yeaa ese 42 | 0.85 | 1.00 | 2.58 | 3.14 | 0.30 | 0.20 | 0.65 |....do. 
1334 | Colorado River. . AQ Schot6:: --< seees| sce ecs| Sesser on ese- 3.60 | 3.60 |...... 0.80 |..-.-- Dry. 
1335 |.----- (Umm sgodeeSanoareee BAG ERS Soe EEE SCO Rese ne Baeeen PoReee 3.30 | 3.30 |.....- Oni 2h} saa .-.do. 
| 1336 |.----- OU hseepeo- a aeeRenocee- Pees) poomentcreesd page| seoce] eee 3.50 | 3.20 |... .- W513) ease meet lie: 
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 deucopus 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 the back by rather 
few blackish hairs. The sides fade insensibly into a pale brownish-yellow, or 
dull tawny cinnamon (almost with a pinkish wash), which reaches down the 
