June 7, 1924 
Variation in Microtus Montanus Yosemite 
993 
THE SKULL 
It is necessary to have some criterion on which to base a consecutive arrange¬ 
ment of the skulls for the purpose of study. The most desirable, of course, 
would be the mean of a variety of characters showing age, such as amount of 
ridging, angularity, and general development of the skull; but it is impossible 
for the human eye to appreciate such fine gradations as are involved, and 
more definite methods must be used. Although some of the smaller skull 
dimensions might be employed with excellent results, it seems preferable to 
make use of one of the longer measurements. The total length of skull is 
perhaps usually favored above all 
others. In some mammals this meas¬ 
urement is an excellent one, but in 
others, whose nasals project sufficiently 
for these to constitute the anterior 
point of the measurement, it is open to 
the serious objection that the fragile 
tips of the nasals are very prone to in¬ 
jury; hence, not only may the skull be 
damaged to this extent, but the subse¬ 
quent measurements will fluctuate with 
the amount of such damage. 
The length of the hensel (distance 
from the inferior lip of the foramen 
magnum to the posterior margin of the 
alveolus of the incisor) is free from this 
objection, and may be an excellent 
measurement to use. As the precise 
relative position of the inferior lip of 
the foramen magnum is apt to vary, 
however, the writer prefers to employ 
the following measurement: 
CoNDYLO-BASILAR LENGTH. -This is 
the distance from the condyle to the pos¬ 
terior margin of the alveolus of the in¬ 
cisor upon the same side. It constitutes 
the criterion upon which the arrange¬ 
ment of the present series is based. As 
previously mentioned, it is the most nat¬ 
ural arrangement, when considered with 
reference to weight and size of the animal 
and its pelage, that could be devised. 
The series of skulls thus arranged exhib¬ 
its a minimum of individual variation. 
The skull as a whole is characterized by no marked peculiarities, and its spe¬ 
cific characters need not be discussed. 
The skull is properly divisible into two parts—the cranium proper and the 
face. The former should be considered as being made up of three rings as follows: 
The posterior ring consists of the occipital. In the middle ring are the inter¬ 
parietal, parietals, basisphenoids and alisphenoids, and thp temporals, including 
the squamosals, mastoids, and audital bullae. The anterior ring is formed of 
the frontals, orbitosphenoids, presphenoids, and the ethmoid. 4 After a fashion, 
each of these three rings forms an integral part and may be said to vary individ¬ 
ually as independently as its articulations with adjoining rings will allow. The 
face is made up of the nasals, premaxillae, maxillae, jugals, lachrymals, pala¬ 
tines, turbinals, 4 and vomer. 4 __ 
* These bones need not here be discussed, as the impossibility of examining them without severely damag¬ 
ing the skull renders them useless as characters in ordinary systematic mammalian studies. 
96461—24f-2 
Fig. 8—Diagrammatic dorsal aspect of skull of adult 
Microtus m. yosemite. a, Anterior nares; 6, pre¬ 
maxilla; c, anteorbital fossa; d, lachrymal; e, 
frontal; /, zygomatic arch; g, postorbital process; 
h, anterior projection of parietal; i, lateral projec¬ 
tion of parietal; j, prelambdoidal fenestrum; k, 
interparietal; l, lambdoidal crest; m, mastoid; n, 
nasal; o, ascending branch of premaxilla; p, max¬ 
illary root of zygoma; q f jugal; r, squamosal; s, 
audital bulla; t, squamosal; u, occipital 
