1922] 



Kellogg: Synopsis of Microius Mordax 



281 



TABLE I — (Continued) 



AGE AND SEX VARIATION 



It would be difficult to determine the sex of any individual from 

 a study of the cranium alone. In practically all instances the range 

 of individual variation is considerably greater than that of sex vari- 

 ation. Thus, in the study of Microtus mordax, sex variation may be 

 in most cases disregarded ; but it is by no means possible to eliminate, 

 altogether, age variation. In most cases, when making comparisons 

 of typical series of adults, it is verj^ difficult to determine what con- 

 stitutes maturity. In a series of fifty specimens of Microtus mordax 

 sierrae, ranging in total length from 190 mm. to 221 mm., it was 

 found on tabulation that there was but slight variation in the condylo- 

 basal lengths of the skulls of these same individuals (see table I). 

 There are two alternative interpretations which can be drawn from 

 these data: either the total lengths as noted by the collector are 

 unreliable, or the crania increase in size very slowly after the animals 

 reach sexual maturity. Of the first interpretation it may be pointed 

 out that errors would be on the average just as much below the true 



