MAMMALS OF THE MEXICAN BOUNDARY. 



395 



with which it must, therefore, be synonymized. I can not agree with 

 Mr. Rhoads in associating- it with the subgenus Baiomys; and the 

 alleged trefoil character of the first upper premolar, upon which the 

 subgenus Trinodontomys was founded, is the 

 result partly of accident, but principally of age. 

 In the accompanying figure (fig. 73) the ante- 

 rior upper molar of three individuals of dif- 

 ferent ages are outlined. In one (a) , the type of 

 insolatus, which is but slightly worn', there are 

 two enamel folds, with the resulting trefoil 

 pattern of the anterior enamel loop; another 

 of Mr. Rhoads's specimens (6) has been worn 

 down to a lower plane, one groove having 

 been ground out, so that this loop appears 

 asymmetrical; a third specimen (c) exhibits a 

 section of this tooth at a still lower plane, all 

 of the cusps having been ground away. The 

 trefoil pattern of the crowii of the first upper 

 molar is commonly seen in youngish specimens 

 of several species of Peromyscus, the pattern 

 ever changing as the cusps and tubercles form- 

 ing the crown of the tooth are gradualy worn 

 away. 



Description. — This is a large, stout, long- 

 tailed, desert race of P. sonoriensis, of which 

 extreme examples are found on the deserts 

 bordering the lower Colorado River. These 

 specimens have massive skulls, thick tails, and stout feet (figs. 74 

 and 75). No. G012] , U. S. National Museum, an old female in winter 

 pelage, taken on the lower Colorado in March, is ochraceous-cinnamon 



Fig. 74.— Pebomtscws sono- 



KIENSIS DESEETICOLA. li, 



hindfoot; 6, tail. 



Fig. Hb. — Peromyscus sonoriensis deserticola. Skull, o, dorsal view; 6, ventral vie"w; 



c, lateral view. 



slightly mixed with black above and pure white below; ears dusky, 

 with lanuginous tuft at base colored like the head. Summer speci- 

 mens are more drab-like. The young are pale drab-gray above, gray 



