996 
Journal of Agricultural Research 
Vol. XXVIII, No. 10 
The interorbital width varies practically none at all with age, and as it is there¬ 
fore proportionately so much greater in young examples, it adds much to the 
short appearance of the skull. Still another contributing 
cause is the weakness of the zygomatic processes of the 
maxillae in very young animals. 
Although the nasals are of practically the same rela¬ 
tive length in the two extremes of age, the rostrum is 
slightly shorter in juveniles, the brevity of which is 
further accentuated by the lack at this age of pronounced 
anterior projection of the incisors. The transverse 
measurement of the superior portion of the rostrum 
between the anteorbital fossae may be actually just as 
great in juveniles, although this apparently is not always 
the case. Where such proportionately greater breadth 
exists it still further lends the appearance of shortness 
to the juvenile rostrum. 
Interiorly, the bullae are relatively slightly larger in 
juveniles, especially in the width of the anterior portion, 
but the pterygoid region is considerably shorter in the case 
of animals of this age. In the two smallest juveniles avail¬ 
able the incisive foramina are located relatively farther 
forward than in adults, which probably means that the 
anterior growth of the premaxillae, as age advances, is 
at a greater rate than that of the maxillae. 
Fig. 12.—Variation of the 
skull with age in Microtus 
m. yosemite: dorsal aspect. 
Left half of the skull of 
adult No. 3835 upon the 
left, separated by medial 
line from right half of skull 
of juvenile No. 3857 upon 
the right; the condylo- 
basilar dimension of both 
magnified to the same size 
Table III .—Measurements of the skulls of males (in millimeters) 
Collection number 
Condylo-basilar 
length 
1 
C3 
1. 
bcS 
tab 
II 
>> o 
r 
o 
Alveolo-basilar 
length 
Condylo-palatal 
length 
Zygomatic width 
Interorbital width 
Lambdoidal width 
Height J 
Tooth row 
Incisive foramina 
Alveolar width 
Nasals j 
Angular process 
3835__ 
28.2 
23.3 
16.5 
13.4 
17.0 
3.7 
13.0 
10.9 
7.2 
5.6 
5.9 
8.0 
6.2 
3837__ 
27.7 
23.0 
16.3 
13.0 
17.3 
3.8 
12.9 
11.2 
7.0 
6.0 
5.7 
8.8 
5.8 
3812_ 
27.6 
22.9 
16.4 
13.1 
17.9 
3.9 
13.5 
10.9 
7.0 
5.8 
5.9 
8.3 
6.2 
3826.__.. 
27.5 
22.9 
16.2 
13.3 
16.7 
3.9 
12.9 
11.2 
7.0 
5.6 
5.9 
8.0 
6.3 
3853... 
27.5 
22.9 
16.3 
12.8 
16.6 
4.1 
12.6 
10.8 
6.8 
5.7 
5.6 
8.6 
5.6 
3825_ 
27.3 
22.9 
16.0 
13.3 
16.8 
4.0 
12.9 
10.8 
6.7 
5.8 
5.7 
8.3 
5.8 
3854.. 
27.3 
22.7 
15.9 
13.0 
17.1 
3.7 
12.5 
11.1 
6.8 
5.6 
5.7 
8.3 
5.9 
3822..... 
27.2 
22.5 
16.1 
12.9 
16.6 
3.7 
12.7 
10.9 
6.9 
5.4 
5.7 
7.7 
5.9 
3818.... 
27.2 
22.5 
16.5 
13.2 
17.0 
4.1 
12.6 
10.8 
7.2 
5.7 
6.1 
8.8 
5.6 
3864.... 
27.1 
22.5 
16.0 
12.8 
16.8 
3.8 
12.9 
11.2 
6.8 
5.8 
5.7 
8.2 
5.5 
3846..__ 
26.1 
21.9 
15.6 
12.3 
16.5 
3.8 
12.4 
10.9 
6.7 
5.2 
5.6 
7.8 
5.6 
3855... 
25.9 
21.6 
15.3 
12.3 
16.2 
3.8 
12.0 
10.8 
6.8 
5.6 
5.7 
7.6 
5.1 
3848... 
25.9 
21.2 
15.2 
12.1 
15.1 
3.7 
12.1 
10.7 
6.4 
5.7 
5.3 
7.3 
5.2 
3862..... 
i 25.7 
21.3 
14.8 
12.1 
15.3 
3.6 
12.3 
10.2 
6.7 
5.0 
5.7 
7.5 
5.6 
3883__ 
25.6 
21.7 
15.1 
12.2 
15.5 
4.0 
12.3 
10.6 
6.7 
5.5 
5.5 
7.5 
5.5 
3820_ 
25.6 
21.3 
15.0 
12.2 
14.9 
3.6 
11.7 
10.2 
6.5 
5.5 
5.4 
7.3 
5.2 
3841--.. 
25.4 
21.2 
14.7 
12.1 
15.4 
3.9 
12.1 
10.4 
6.5 
5.5 
5.5 
7.0 
5.4 
3870... 
25.1 
20.9 
14.8 
11.2 
15.6 
4.0 
12.2 
10.8 
6.6 
5.2 
5.5 
5.6 
3847... 
24.9 
20.7 
14.8 
12.0 
15.2 
3.8 
12.2 
10.5 
6.3 
5.2 
5.3 
7.3 
5.4 
3867_ 
24.7 
20.9 
14.6 
12.3 
14.7 
3.9 
11.9 
10.7 
6.5 
5.3 
5.4 
6.9 
5.5 
3887... 
24.5 
20.3 
14.3 
11.4 
14.4 
3.6 
11.7 
10.2 
6.1 
6.4 
5.4 
6.9 
4.6 
3814.... 
24.4 
20.5 
14.3 
11.4 
15.0 
3.7 
11.9 
10.2 
6.2 
5.2 
5.4 
7.0 
4.7 
3849___ 
24.3 
20.5 
14.3 
11.2 
15.3 
3.7 
12.1 
10.4 
6.4 
5.2 
5.5 
7.0 
5.0 
3801.. 
23.4 
19.8 
14.2 
11.2 
14.1 
3.5 
11.4 
9.7 
6.2 
4.9 
5.3 
7.0 
4.8 
3857.... 
22.7 
19.3 
13.5 
10.6 
14.4 
3.6 
11.2 
10.1 
5.9 
4.7 
5.2 
6.5 
4.6 
Averages.. 
26.2 
21.8 
15.45 
12.45 
16.0 
3.84 
12.4 
10.7 
6.68 
5.5 
5.6 
7.74 
5.6 
a Average measurements of 19 (roughly, three-quarters) of the skulls of males, the largest two and 
smallest four specimens being eliminated from consideration. 
