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University of California Publications in Zoology [Vol. 21 



that area of the latter is still darker. In short, pacificu^ is a light- 

 colored, "yellow-sided" form, while montamis is a dark, "buff -sided" 

 form. The skulls of the two species are equal in size and in length 

 of rostrum and nasals, but the rostrum of pacificus is more tapering, 

 the bullae are smaller, and the incisive foramina are slightly narrower 

 and more constricted anteriorly. The brain-case is slightly less 

 expanded, and the upper tooth rows converge posteriori}' instead of 

 being parallel as in montanus. I am unable, however, to distinguish 

 even the slightest difference in the interpterygoid fossae (see Merriam, 

 1897, p. 104). 



Fig. 1. Distributiou in California of the species and subspecies of Zapus. 



