June 7, 1924 
Variation in Microtus Montanus Yosemite 
1003 
Fig. 18—Indi¬ 
vidual varia¬ 
tion of the in¬ 
terparietal in 
the skulls of 
adults, a, No. 
3817; 6, No. 
3815; c, No. 
3836 
angle is sometimes formed, however, or an illy defined double curve. There is 
no definite variation with age. 
Parietals. —Anteriorly this pair of bones articulates with the frontals, 
laterally with the squamosals, and posteriorly with the interparietal. Antero- 
medially there is a recession of the parietals, filled by a part of the frontals, and 
each has an anterior projection and a lateral one. These projections and reces¬ 
sions are least developed in juveniles, but after the attainment of sexual maturity 
there is negligible variation with age, though much individually. 
This is more pronounced in the anterior than in the posterior pro¬ 
jection, though there are a few exceptions to this rule. 
The temporal ridges are continuations, almost at right angles, of 
the lambdoidal crests. From the latter they cross the extreme lat¬ 
eral borders of the interparietal, extend across the bases of the 
lateral projections of the parietals, and almost always follow the 
borders of the anterior ones to their antero-medial tips, thence 
joining the interorbital ridging. Occasionally, however, the tem¬ 
poral ridges cut across a part of the anterior projections. Differ¬ 
ences in their precise positions depend almost altogether upon 
similar differences in the shape of the parietals. The degree to 
which these ridges are developed is dependent upon the develop¬ 
ment of the temporal muscles. Therefore the strongest, most 
angular skulls have the best-defined temporal ridges, while in weaker, 
rounded skulls with narrow zygomatic width they are poorly 
defined. They may be lacking or quite distinct in medium-sized 
subadults, but in individuals which are fully adult they are 
always apparent, though in varying degree. On the whole, this character is 
perhaps better defined in males, although there are female skulls that are fully as 
heavily ridged. 
Squamosals. —Each squamosal articulates posteriorly with the occipital and 
mastoid, medially to a slight extent with the interparietal as well as with the 
parietal, anteriorly with the frontal, and laterally with the alisphenoid and 
mastoid. The jugal articulates with the zygomatic root of the squamosal. 
Perhaps the chief variation in the outline of this bone lies contiguous to the 
anterior projection of the parietal. In some individuals, espe¬ 
cially the youngest ones, the squamoso-frontal suture begins at 
a point upon the antero-lateral margin of this projection, while 
in the majority it is situated relatively more medial. It is also 
influenced by the size of the anterior projection of the parietal. 
The squamosal-interparietal suture differs individually, but not 
with age, in extent and exact shape as pointed out in the 
discussion of the interparietal. The posterior border of the 
squamosal constitutes the anterior face of the lambdoidal crest 
and immediately anterior to this is situated the prelambdoidal 
fenestrum of the squamosal. This is not a true foramen in the 
bone, but rather seems to have been formed in remote times by 
an invagination of the bone from its posterior border, the pos¬ 
terior edges of the fenestrum having joined at a later period, the 
resulting false suture being plainly distinguishable in juveniles 
but disappearing with age in this species, although it is not 
so lost in some other genera of this subfamily. The exact size 
of this fenestrum is highly variable individually, in length 
about 20 per cent. Its infero-lateral border is formed of a slender bar of bone, 
and infero-laterally to this is the premastoid vacuity, usually well defined but 
occasionally practically obliterated. 
c 
Fig. 19—Individual 
variation of the 
parietals in adult 
skulls, a, No. 3815; 
6, No. 3863; c, No. 
3864 
