June 7, 1924 
Variation in Microtus Montanus Yosemite 
1013 
In the first group the outer lobe of the trisection is situated practically entirely 
posterior to the inner. All loops remain open, and each loop may vary widely 
in size and proportions. In this group, represented by figure 25, a, are 75 of the 
132 rami. 
In the second group, of which figure 25, b is typical, the outer lobe of the tri¬ 
section is not practically entirely posterior to the inner, but in only a few cases is 
it situated just as far forward as the inner. All loops remain open in this group 
also, in which belong 48 of the rami. 
Nine rami are assigned to the third group, illustrated by figure 25, c and g. 
In it the outer lobe of the trisection is situated posterior to the inner one and is 
entirely closed, forming a supplementary triangle upon that side of the tooth. 
In the remaining details of figure 25 are shown the more noteworthy variations 
of these three patterns. 
e f g h 
Fig. 25.—Individual variation in the pattern of the first lower molar. MlJ: a, No. 3888; d, No. 3873; 
|Ml: 6, No. 3850; c, No. 3820; e, No. 3842; /, No. 3853; g, No. 3825; h, No. 3863 
The writer is convinced that this great variation in. tooth pattern within the 
series is without significance in the present connection, save, as in the case of the 
third upper molar, that it indicates evolutional instability at this point. As 
previously mentioned, additional subgroups might be designated in order to 
classify the variants with still finer exactness, but as the differences are purely 
individual and without connection with other characters, in so far as it is possible 
to see, they need not be dwelt upon in greater detail in the present paper. 
The first lower molar, then, consists of a highly variable, tripartite division 
anteriorly, followed by two closed triangles upon the outer or buccal side and 
three upon the inner or lingual side, with a posterior crescentic loop. All these 
triangles are normally closed, exceptions occurring in two cases in which the 
anterior inner triangle remains open (fig. 25, /). 
Second molar. —This consists of two small outer triangles, two larger inner 
ones, and a posterior crescentic loop. There is no appreciable variation. 
Third molar. —In this tooth there are three simple, closed loops, the anterior 
one smallest and the other two progressively larger. The inner reentrant angles 
are deep, but the outer ones are barely indicated. There is no appreciable 
variation. * 
