1922] Kellogg: Calif or nian Forms of Microtus Montanus 255 



from one type to another in whatever direction one might wish to 

 illustrate it, except in case of certain abnormal or aberrant variations 

 snch as is shown in figure P. 



Q 



Figs. Q-T. Diagrams of enamel pattern of second upper molars of Microtus 

 montanus yosemite. X 6. Fig. Q, no. 22361, 5, Mus. Vert. Zool., Tuolumne 

 Meadows, Yosemite Pai'k; fig. E, no. 24012, $, Mus. Vert. Zool., Mono Lake 

 (Salmon Eanch), Mono Oounty; fig. S, no. 22363, $, Mus. Vert. Zool., Tuolumne 

 Meadows, Yosemite Park; fig. T, no. 23988, c?, Mus. Vert. Zool., Farrington's 

 Eauch, Mono Lake, Mono County. 



Figs. U-X. Diagrams of enamel pattern of second upper molars of Microtus 

 montanus dutcheri. X 6. Fig. U, no. 15601, 5, Mus. Vert. Zool., Whitney 

 Meadows, Tulare County; fig. V, no. 15619, c?, Mus. Vert. Zool., Whitney Creek, 

 Tulare County; fig. W, no. 15622, ?, Mus. Vert. Zool., Whitney Creek", Tulare 

 County; fig. X, no. 15595, $, Mus. Vert. Zool., Whitney Meadows, Tulare County. 



Considerable amplitude is to be allowed in interpreting the cate- 

 gories of variations as shown by table III. In some instances the assign- 

 ment of certain specimens to any particular type was arbitrary, though 

 in most cases the particular tooth in question was clearly a modifica- 

 tion of some one of the regular types of variation. Of the second 

 upper molars, figure L represents the mean, and of the fourth lower 

 premolars, figure F is the type most often present. No two fourth 

 lower premolars are exactly alike. However, there are fairly definite 

 categories of these variations, which group themselves around certain 

 types as illustrated by table III, with especial reference to figures E, 

 F, G, H, I, and J. 



Figures showing the' molariform series of Microtus calif ornicns 

 with tritubercular nomenclature are included in the present paper 

 in order that reference may be made to it in later studies, and that 

 it maj' be available for use of other workers on that group. 



