160 University of California Publications in Zoology [Vol. 10 



to Buena Vista and Kern lakes. It should be noted that the 

 representatives of the genera Dipodomys and Perodipus, as well 

 as of the subgenus Perognathus, are different on the McKittrick 

 and Bakersfield sides of the slough. On the other hand, the 

 species of the genera Citellus and Ammospermophilus, and of 

 tin- subgenus Chaetodipus, of Perognathus, are the same on the 

 two sides. These facts would indicate that while the barrier in 

 question is not highly efficient, taking all the forms into con- 

 sideration, it may have been of some importance in the forma- 

 tion of the McKittrick pocket-mouse. On the other hand it 

 should ho noted that specimens of longimembris from Huron, 

 Fresno County, west of the San Joaquin River farther north- 

 ward, were examined by Osgood, and that examples of longi- 

 iiii mbris from Tracy and Los Banos, also west of the river, are 

 not appreciably different from those from districts east of the 

 stream. This, of course, militates against the possibility of 

 the river's being a harrier of sufficient importance to be a con- 

 dition in the divarication of the two forms. 



Large, dark races of a species are sometimes found in the 

 humid coast belt, when small, light forms occur in the arid in- 

 terior. For example, in northern North America, the smaller, 

 lighter subspecies of /'< romyscus maniculatus (see Osgood, 1909), 

 namely nebrascensis. anticus. and arhmisiac, occur in the more 

 arid interior regions, while the larger, darker ones, oreas, hyla< us, 

 keeni, macrorhinus, and the closely related sitkensis and prevos- 

 h iisis, are found in the more humid coast districts. The new sub- 

 species of Perognathus here described, which occurs in a locality 

 more arid than that in which Perognathus I. longimembris is 

 found, is paler, but also larger, which condition is contrary to 

 what might be expected from the contemplation of such a case 

 as that cited above. 



In the diagram of the distribution of variation, specimens of 

 Perognathus I. longimembris are represented by Arabic numerals 

 placed in the upper portions of the squares, and in the frequency 

 curves by black dots and solid lines. Perognathus I. neglectus, 

 on the other hand, is indicated by Roman numerals placed in 

 the lower portions of the squares, and in the frequency curves 

 by open circles and dotted lines. 



