246 UmversUy of California Publioations in Zoology [Vol.21 



DIAGNOSTIC CRITERIA 



The diagnostic criteria that appear to be best adapted for dis- 

 tinguishing- the several races of Microtns montanns have been deter- 

 mined by assorting the various characters into certain categories, and 

 ascertaining the mean manifestation with respect to each character. 

 It is a conspicuous fact that the three races in question do overlap 

 to a considerable extent in respect to many characters. Specimens 

 of this group frequently cannot be assigned to any particular sub- 

 species unless the locality at which they were collected be known. 



The diagnoses of the crania describe the average, or the mean, of 

 all individuals examined of any particular race rather than any 

 particular type or individual. Color, in the Microhcs montanus group 

 at least, is too unreliable a feature to be of much use in separating 

 the different races in California. Therefore the color terms used in 

 the following descriptions relate to extremes, not to averages; it is 

 impossible to designate any particular shade as the average, because 

 there is such wide variation in color in each race. Furthermore, many 

 of the shades to be seen in a.n.y one subspecies can be matched in 

 individuals of a different form. 



VARIATION 



There is evidenth' a wide range of individual variation within each 

 subspecies with respect to color, external measurements (i.e., total 

 length, length of tail, length of hind foot, and height of ear above 

 crown), enamel pattern of the molariform series, and configuration 

 of the skull. The variation within a given subspecies is frequently 

 as great as the differences between the means of two subspecies. 



In view of the emphasis laid on cranial characters as a means of 

 differentiating closely allied subspecies, it is well to point out the sort 

 of variation that takes place in the configuration of the skull and in 

 the form of the separate bones. In general, the contour of the brain 

 case, the relative size and shape of the interparietal and frontals, and 

 the development of the interorbital ridges are altered materially as 

 the animal reaches maturity; and as old age approaches, the ridges 

 and tuberosities, the angulation of the zygomata, and other peculiar- 

 ities of the skull become more and more accentuated. In young 

 individuals the occipito-nasal length is usually greater than the con- 

 dylobasal length ; as the animal reaches maturity and approaches old 

 age, there is a gradual shifting forward of the brain case until finally 

 the reverse is often true. In skulls of young individuals, the brain 



