100 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
rather fall under the sonoriensis category by reason of the shortness of the 
tail. There is little to note regarding these specimens. No. 7186, from Fort 
Mojave, is about the palest fulvous all over the upper parts we have seen, but 
still is not at all like eremicus. No. 7183 is almost a typical /eucopus. The 
two adults of the Tejon series have almost exactly the body-colors of gossy- 
pinus ; and we should not omit to note that they have a suspicious sort of 
resemblance to H. californicus. Nor should we be surprised if some mice 
from this vicinity should be found to bridge over the chasm that now appears 
between the /eucopus styles and the ‘seemingly very different ‘‘californicus”’ ; 
one, at least, of the Tejon specimens (No. 3670) being veritable californicus. 
We have already noted that Northern Californian mice, and more particu- 
larly Oregon and Washington ones, are darker than those of Southern Califor- 
nia (in this respect precisely matching Massachusetts deucopus), and that 
they shade directly into austerus in this respect, as well as in the length of 
the tail. 
HESPEROMYS (VESPERIMUS) AZTECUS, De Saussure. 
Aztec Mouse. 
Hesperomys aztecus, DE Saussure, R. M. Z. 1860, 105, pl. ix, f. 4 (teeth). 
THesperomys (Vesperimus) aztecus, Cours, Proc. Acad. Nat. Sci. Phila. 1874, 180. 
Diacnosis.—H1. leucopo staturd par, et formd simillimus, sed caudé sub- 
nuda et coloribus differt. Brunneus, dorso nigricante, lateribus ferrugineis, 
partibus inferioribus albis ; antebrachio extus later:bus concolore, crure et parte 
tertia basali metatarsi fuscescentibus; caudd fere unicolore. 
Hanrrar.— Mexico” (De Saussure). 
This species belongs strictly to the Hesperomys leucopus group,* and does 
not differ noticeably in size or proportions from that species. The coloration, 
however, appears specifically distinctive; it differs from that of the ordinary 
United States species precisely as Neotoma ferruginea differs from floridana. 
The upper parts are rich rusty-red, almost orange rust-color, on the 
back deepening into a broad area of brownish-black, and only showing in its 
intensity on the sides where the line of demarkation with the white of the 
under parts is distinct and sharp. The head shares somewhat of the general 
blackening of the middle area above, but is not so uniformly dark as the back ; 
the extreme muzzle is dark, leaving about the mouth only the sides of the 
*In all external features, at any rate; but we have not ascertained the existence of cheek- 
pouches. 
