226 University of California Publications in Zoology [Vol. 21 



Two hundred and eighty-six specimens of Zapus have been exam- 

 ined from forty-eight localities within the state of California, as well 

 as small series of some of the forms from north of this state. 



GENERAL DISCUSSION 



The cranial differences which obtain between the forms of Zapus 

 are not great, and there is seldom anj^ one diagnostic character which 

 makes identification possible without further comparison. The amount 

 of individual variation in the skulls, even of topotypes of well marked 

 species, is often considerable, but this is taken fully into account in 

 defining the species and subspecies. In spite of the minor differences 

 in any one series, there are usually several constant characters which, 

 taken together, make final determination entirely satisfactory. 



There is available no series consisting of specimens taken in a 

 single locality in all months during which the animals are active. 

 This is a misfortune, for seasonal variation between spring and fall 

 is considerable in some forms. However, this difference seems to be 

 entirely due to fading and wear. Although none of the skins are in 

 new pelage, it is assumed that there is only one annual molt, and that 

 the new coat is acquired some time after the middle of September. 

 Too much stress should not be laid on the exact shade of coloration 

 within a series unless the material at hand be strictly comparable as 

 to age and stage of pelage. Nor, of course, should too much confidence 

 be placed in the measurements in the flesh made by different collectors, 

 because of difference in method of measuring. 



There is seemingly no one character which can be considered as 

 more important than any other. All parts of the cranium must be 

 carefully considered. Rostrum including nasals, brain-case, incisive 

 foramina, bullae, and the pterygoid fossae, all present features of 

 value. The proportion of black to lighter hairs on the dorsal area is 

 variable, but the tone of coloration of the sides and underparts is of 

 importance. Immatures are usually a trifle paler dorsally than are 

 adults. 



Coloration seems to have been less easily affected by evolutionary 

 influences than the cranial characters, and by following this more 

 conservative criterion, of color, it is found that California species fall 

 into three natural groups, inhabiting three main geographic areas. 



