396 



BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



and white below. Topotypes are smaller and grayer (less ochraceous) 

 than the extreme Western Desert phase described above. In fact, 

 they combine the characters of medius and sonoriensis with those of 

 the form from the true (southern) Western Desert Tract, the type- 

 locality having been badly s'elected; but the name deserticola can very 

 well be made to cover the whole Western Desert Tract, as the type 

 is nearest to the form occupying this area. From sonoriensis it 

 differs in having a slightly larger foot, ear, and tail; in being more 

 ochraceous; and in having the skull appreciably higher over the 

 orbits. The teeth are shown in fig. 76. 



Habits and local distribution. — No plains mice were met with in the 

 dry region lying between the Santa Cruz Valley until we came to the 

 Gila River at Adonde, Arizona, in February, 1904. 

 It is much the commonest mammal of the lower 

 portions of the Gila and Colorado rivers. To the 

 rule that it is found only in damp situations there 

 are some exceptions; but it is a species of meadows 

 and river bottoms. Nowhere did we find it so 

 abundant as on the vast savannas of the Colorado 

 River about the head of the Gulf of California, where 

 every trap set amongst the canes and hemp, or in 

 the coarse grass, was pretty certain when next 

 visited to contain one of these mice, which are both 

 diurnal and nocturnal. Many cotton rats trapped 

 at the same time were eaten by this mouse; but so neatly was 

 the meat extracted through a small opening in the abdomen that 

 many of the rats were made into very good skins. On the Colorado 

 Desert mammal life became more and more scarce as we went west- 

 ward, until, at last, it was hardly worth while to put out the traps. 

 Seven pregnant females were taken from March 16 to April 16, the 

 fetuses numbering as follows: 4, 4, 5, 4, 3, 3, and 3, the average 

 number of young being about 4. Besides insects, quantities of 

 Xanthium seeds are eaten by this mouse. 



Fig. 76.— Peeomyscus 

 sonoriensis deseb- 



TICOLA. CEOWNS of 

 MOLAR TEETH. a, 



UPPER series; &, 



LOWER SERIES. 



