914 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
Arctomys monax differs from A. flaviventer, its nearest ally, in its smaller 
size, relatively much shorter tail, larger ears, and quite different coloration. 
The style of coloration is nearly the same in the two species, both being 
grizzled above, through the gray tipping of the hairs, and more or less varied 
with fulvous. A. flaviventer is characterized by a more golden hue and by 
the subterminal bar of the long hairs being reddish-brown instead of black. 
A. monax lacks the pale yellow color of the buttocks seen in A. flaviventer, 
and the whitish-yellow area on the sides of the neck. The feet and tail are 
also black instead of yellow or yellowish-brown. The difference in size is 
quite marked, A. monax averaging about three inches shorter in the head and 
body length, while the tail is also three inches shorter than that of A. flani- 
venter, and hence relatively much shorter. Still more strongly marked differ- 
ences are seen in the skulls. In A. monax, as compared with A. flaviventer, 
the palate is broader, the molar series farther apart and strictly parallel, the 
teeth themselves larger, and the palatal surface smooth ; whereas in the latter 
the molar series are slightly convergent posteriorly, the palatal surface is nar- 
rower, absolutely as well as relatively, with a rather deep groove on each side, 
extending usually from the maxillo-intermaxillary suture to the last premolar, 
and sometimes considerably beyond this point; the posterior nasal opening 
is also narrower, and the pterygoid processes more convergent. In A. monaz, 
the skull is broader in proportion to its length, the nasals are longer, and the 
frontals extend further forward, resulting in a much smaller exposure of the 
maxillary in view from above. The anterior face of the zygomatic process 
of the maxillary is also much narrower than in A. flaviventer. As compared 
with A. pruinosus, all the differences, both cranial and external, are far 
greater than between A. morax and A. flaviventer, rendering a detailed com- 
parison unnecessary. In respect to size, A. pruinosus must be an animal of 
twice the bulk and weight of A. mona. 
As may be inferred from the foregoing remarks, A. monaz presents a 
wide range of individual variation in color. It also varies greatly in respect 
to the characters of the skull, particularly in respect to the length and form 
of the nasal bones, and the breadth and posterior extension of the premaxil- 
laries. When the nasals are greatly narrowed posteriorly, the intermaxillary 
becomes proportionally widened, its width opposite the zygomatic process of 
the maxillary being fully twice as great in some specimens as in others. The 
adult skull, among specimens from the same locality, varies in length from 
