MAMMALS OF THE MEXICAN BOUNDARY. 



447 



grayish hairs on inner surface, with their convex surface black ante- 

 riorly and buff posteriorly; tail brownish black, somewhat lighter 

 below, the hairs almost conceahng the annuli. 



Cranial characters.— Tan skull (fig. 110), compared with skulls of 

 Sigmodon hispidus and its races, is short and high, with the rostral 

 portion shortened and the nasals more contracted apically. Corre- 

 lated with the greater height of the brain case is the much deeper 

 temporal fossa, which is divided by an approximately median lon- 

 gitudinal osseous ridge into two nearly equal areas, the lower giving 

 rise to that portion of the temporalis muscle which is inserted into 

 the outer surface and posterior margin of the coronoid process of the 

 mandible, the upper to that which is inserted in the anterior border 

 and inner surface of the coronoid. In Sigmodon minimus these two 

 parts of the temporalis muscle are of approximately equal size, while 

 in S. hispidus the two are very unequal, the lower being much smaller 

 than the upper. 



Fig. 110. — Sigmodon minimus. Skull, a, doesal view; &, ventral view; c, lateral view. 



General remarlcs. — The original description of this species, written 

 in the field, was based on two skins and three skulls, together with 

 several fragments, all from the tyjje-locality. At the present time 

 thirteen additional skins from localities farther west, beyond the Sail 

 Luis Mountains, are before me. These, allowing for season, are 

 darker in color and somewhat larger. Dr. J. A. Allen has given the 

 measurements of two adult males, taken at San Bernardino Ranch 

 (Monument No. 77), by Mr. B. C. Condit, and these agree very closely 

 with the dimensions of our specimens from the Santa Cruz Valley, 

 near Monument No. 111. As the type of Sigmodon minimus was 

 obtained at the western edge of the Eastern Desert, it is possible, 

 should its range be found to extend into the center of that tract, that 

 the larger and darker central form may require separation as a geo- 

 graphical race, with the Elevated Central Tract for its habitat. The 

 typical form in that case would be restricted to the Eastern Desert 

 Tract. For comparison with the type and topotypes of Sigmodon 

 minimus, two adults, male and female, taken at Igo's Ranch, at the 



