June 7,1924 
Variation in Microtus Montanus Yosemite 
1001 
Measurements of height of the rostrum are not herejpresentedjfor the reason 
that it is impossible to obtain them with any sort of uniformity. This, neverthe¬ 
less, is a character of great importance in specific diagnosis. Individual varia¬ 
tion is great, especially in the height of the posterior portion of the rostrum. 
Maximum height, or depth, of this member is practically always accompanied by 
pronounced declivity of the supero-lateral profile. In other words, great depth 
of the skull is accentuated in this region and but little or not at all in the anterior 
portion of the rostrum. There is equal variation in its ventral profile; in some 
this is almost straight, while in others, chiefly those with considerable depth of 
this member, it exhibits pronounced concave curvature. Although as a rule 
there is no great difference in the relative lengthfofjjthe rostrum, several indi¬ 
viduals have rostra that are conspicuously stubby, and this is correlated only 
partially with length of the nasals. Female No. 3827 is especially noteworthy 
in this respect, as an examination of its condylo-basilar, condylo-zygomatic, and 
nasal measurements, in comparison with those of No. 3876, for instance, will 
show. 
BONES OF THE SKULL 
Occipital. —This bone is divided into three areas, the supraoccipital above, 
the exoccipitals laterally, and the basioccipital below. Superiorly the supra¬ 
occipital articulates with the interparietal and squamosals; 
the exoccipitals with the mastoids; and the basioccipital, 
skirting the auditory bullae, anteriorly with the basisphenoid. 
To the occipital bone are attached many of the powerful 
cervical muscles, and its configuration therefore reflects the 
degree of their development. A pronounced tilting forward 
of the occipital is acknowledged to be a modification brought 
about by fossorial habits and is supposed to have been caused 
by the stresses brought upon the bone and its attached 
muscles by the animal pushing aside the earth of the burrows 
with its head. Incidentally, the writer is not at all convinced 
that other life habits besides fossorial ones do not bring about 
somewhat similar results. At any rate, within the present 
series there is excessive variation in the forward inclination 
and all other aspects of the occipital when viewed vertically 
from above. In fact, hardly any two examples are precisely 
alike in the shape of the superior border and the irregularities 
of the posterior surface of this bone. These variations are 
present irrespective of age and are purely individual, the result of variation in the 
cervical musculature. There is similar variation in the visibility of the condyles 
when the skull is viewed from above; in some specimens they can not be seen, in a 
few they are prominent, but in the majority they are barely perceptible. This 
difference, largely individual, is usually caused by the configuration of the supra¬ 
occipital, but is also due to the fact that in the younger animals the condyles 
are shorter. There is like difference in the exact shape and size of the foramen 
magnum, irrespective of age. 
The superior borders of the lambdoidal processes of the occipital form the 
posterior faces of the lambdoidal crests. The degree to which these are de¬ 
veloped depends upon the strength of the temporal muscles which are attached 
to them, and therefore their prominence is also correlated with the temporal 
ridges and to only a slightly lesser degree with the angularity and strength of 
the whole skull. There are lambdoidal crests of respectable size, however, in 
Fig. 17.—Individual 
variation in depth of 
rostrum and declivity 
of its dorsal outline in 
adult skulls. No.3865 
above; No. 3826 below 
