914 MONOGRAPHS OF NORTE AMERICAN EODENTIA. 



Arctomys monax differs from A. Jlaviventer, its nearest ally, in its smaller 

 size, relatively much shorter tail, larger cats, 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. Jlaviventer 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 t lie pale yellow color of the buttocks seen in A. Jlaviventer, 

 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. Jlavi- 

 venter, and hence relatively much shorter. Still more strongly marked differ- 

 ences are seen in the skulls. In A. monax, as compared with A. Jlaviventer, 

 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. monax, 

 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. Jlaviventer. As compared 

 with A. pruinosas, all the differences, both cranial and external, are far 

 greater than between A. mor.ax and A. Jlaviventer, rendering a detailed com- 

 parison unnecessary. In respect to size, A. pruinosus must be an animal of 

 twice the bulk and weight of A. monax. 



As may be inferred from the foregoing remarks, A. monax 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 proport ionally widened, its width opposite the zygomatic process of 

 t he 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 



