June 7,1924 
Variation in Microtus Montanas Yosemite 
983 
MOLT 
None of the specimens at hand is conspicuously in molt. In fact, a definite 
zone of molt such as often occurs in the case of Thomomys seems never to occur 
among microtines. The same applies to the writer’s belief that there is no very 
definite season for the change of pelage in this subfamily. Individuals appar¬ 
ently molt largely according to age, although there are presumably other factors, 
chiefly climatic or dietetic, which may accelerate or retard such changes. About 
the time of sexual maturity there is usually a lengthening and slight brightening 
of the pelage, apparently due to a gradual growth of new hairs. This seems to 
begin upon the lower sides and extends upward. In some individuals it is also 
seen to extend from the face rearward- but not at all pronouncedly. In the 
specimens that are most worn there is no evidence of the 
growth of new hairs, possibly indicating that such worn con¬ 
dition of the coat is not due to an imminent approach of the 
molt, but rather to some mechanical agency, or possibly to 
some pathological condition of the individual. Unfortunately, 
the writer failed to record, except in the case of a very few 
of the last specimens, the condition and color of the inner 
surface of the pelt. Such observations might have proved to 
be of decided interest. 
MAMMAE 
All females had eight mammae, there being two pairs in¬ 
guinal and two pairs pectoral. Some of these, usually the 
pectorals, were often extremely hard to distinguish in the 
smaller subadults. It is a well-known fact that not all the mam¬ 
mae may be functional in the case of young animals with small 
litters; hence the full complement may be correspondingly 
difficult of definition. 
PLANTAR TUBERCLES 
Fig. 1—Sole of the left 
It seems to have been accepted for years without serious hind foot of Microtut 
question that species of the subgenus Microtus uniformly and m.yosmto(No.3858), 
invariably have six tubercles or pads upon the sole of each th^lanta^ads 11 ° f 
hind foot. The writer has found that there is much variation 
in this respect in the case of a number of species, though not all, and variation 
in the present form proves to be great. 
Save in the case of spirit specimens, satisfactory determination of the plantar 
pattern must be made with fresh material. It is thus possible to compare only 
such unprepared animals as may be at hand, and it is impossible precisely to 
carry in one’s mind the exact plantar development of the individuals in a con¬ 
siderable series. Hence, it is extremely difficult to record in satisfactory terms 
the variations which may Qccur. In the present instance the error must have 
been appreciable in judging the precise amount of development, whether one 
should say there were five or six pads upon an individual foot according to 
whether the sixth was vestigial, small, or medium. 
There is considerable individual difference in the development of the tubercles 
which is entirely independent of age. In some animals they are relatively 
swollen and well developed while in others they are smaller, though more sharply 
defined as a usual thing. The general plan consists of a plantar pad upon the 
sole between, but posterior to, the bases of each pair of toes—four of them, which 
are designated the anterior ones. In addition, there is situated posterior to the 
first or antero-medial tubercle—the one between the bases of the hallux and second 
digit—a fifth tubercle, the postero-medial, which is normally the largest of all, 
but is variable to some extent. The usual sixth tubercle, the postero-lateral^ 
