1014 
Journal of Agricultural Research 
Vol XXVIII, No. 10 
SUMMARY 
This paper embodies the results of an intensive study of a large series of a 
meadow mouse, Microtus montanus yoSemite Grinnell, undertaken to secure 
data on the amount of variation to be found in a single species of small mammal 
represented by specimens taken at the same place, within a short period, and 
by the same collector, thus insuring uniformity as regards preparation of mate¬ 
rial. It is believed that the results will furnish a criterion useful to later students 
of related groups of small rodents. 
The series under discussion, consisting of 66 skins with skulls, besides spirit 
specimens, was collected at virtually one time and at a single spot, in Mono 
County, California. The conditions existing there were very uniform, and the 
locality unusually free from disturbing influences. 
Coloration, especially of the females, is somewhat variable. Much of the 
difference in the appearance of the pelage of males is due to fighting and to 
varying development of the hip glands. The females may be divided into a 
grayish group, in which are most of the large adults, many of which are grayer 
than any male; and a brownish one, consisting largely of subadults. Juveniles 
have a pelage that is more compact and slightly darker. 
The molt is insidious. Four females are excessively worn, but there are no 
males in this condition. 
The plantar tubercles are variable in size and number, but five is the usual 
complement. Hip glands vary in development, those of old females being of 
about the same size as in subadult males. They are absent in juveniles and are 
probably dependent upon sexual development. There are eight mammae present. 
The measurement of total length is subject to considerable error, and the 
variation in sexually mature animals is great. The tail not only varies much in 
length, but it is especially subject to injury during the life of the animal. The 
hind foot attains full growth at an early age, and its length, although varying 
to some extent, is a reliable character. 
Weight varies a great deal, especially in the case of females, as the latter often 
contain embryos, but it is very valuable as an index to the general development 
of the animal. Young females are more often found to be pregnant than old 
ones, but with the latter the litters average larger. 
Most of the individual variation of the skull is due to differences in the devel¬ 
opment of the muscles of mastication, and no thorough understanding of the 
osteology can be gained without some knowledge of the myology. Bones to 
which the main muscles are attached vary in response to stimulation by the 
latter. Other bones vary because of crowding. 
The skull consists of the face and cranium proper, the latter being divisible 
into three rings, the anterior one of which is “dead center” of the skull. Skulls 
of juveniles are much weaker and more rounded than those of adults, and this, 
with lack of ridging in the former, accentuates the appearance of age difference 
to a greater extent than is actually found to be the case. The capacity of the 
braincase is, to all intents, proportionately the same at all ages. 
The measurement of condylo-basilar length is the most satisfactory criterion 
for the arrangement of a series of skulls with respect to general development. 
Other composite measurements are of value as indices to the variation existing 
in divers portions of the skull and to indicate correlated trends. They vary in 
different degrees, both individually and with age. Variation in the precise 
configuration of the bones of the skull is, of course, infinite, such being the result 
of an enormous number of the most complicated developmental factors, only a 
very few of which can ^>e partially defined. 
