1922 J Kellogg: Calif or-nian Forms of Microtus Montanus 247 



case is disproportionately large in comparison with the rostral and 

 palatal regions of the skull. 



Certain variations occur in skulls of the same sex and of approx- 

 imately the same age among mature individuals of the same subspecies. 

 There appear to be but few elements of the adult skull that are con- 

 stant, either in form or in relative size as compared to other parts. 

 Thus the interparietals are extremely variable in appearance. The 

 posterior border may be nearly straight or may exhibit many degrees 

 of convexity, from a type that is gently convex to another in which 

 it is nearly acutel^y convex. The lateral terminations may be either 

 straight, rounded, emarginate, or sloping, and the median projection 

 is variable with respect to size and to length. The posterior termina- 

 tions of the nasals ma.y be attenuate, emarginate, truncate, or even 

 rounded. The interorbital ridges vary from a type in which there is 

 a distinct sulcus between them to one where but a single interorbital 

 crest is discernible. 



By selecting specimens near the same age, thus eliminating those 

 variations which are the result of different stages of growth, a fairly 

 reliable series was obtained for comparative study. Specimens of 

 Microtus montanus montanus from Sisson, California, old and young 

 adults, were chosen to illustrate in graphic form (see tables I, II, 

 and III) the extent of variation that might be expected in any par- 

 ticular locality, although, of course, some age difference is also repre- 

 sented. In general, total length indicates the age very closely, as a 

 critical study of the skulls shows that the oldest individuals are, as a 

 rule, the largest. 



Table I shows that there is little or no correlation between length 

 of tail and length of hind foot. This table probably contains inaccu- 

 racies. The specimens were obtained and the measurements taken 

 and recorded by several collectors, employing different methods. The 

 animals themselves were by no means all in the same condition, some 

 being freshly killed, in some rigor mortis had set in, and some had 

 been dead so long as to be altogether relaxed. The data for tables 

 II and III, however, were prepared by one person, the writer, and 

 he was carefully consistent in the methods employed. 



It will be observed from table II that the mastoid width increases 

 very little with general growth. The condylobasal length, as pre- 

 viously noted, increases during growth in correlation with changes 

 in the cranium. However, no such correlation is apparent between 

 the types of enamel folding exhibited by the second upper premolar 

 and the fourth lower premolar (see table III). 



