June 7,1924 
Variation in Microtus Montanus Yosemite 
1009 
as to be almost pointed. It is in width, however, that this portion of the nasals 
chiefly varies, this difference in extremes amounting to fully 100 per cent. There 
is normally a slight lateral constriction at about the center of these bones, this 
being more pronounced in a few and absent in several others. The degree to 
which the anterior terminations of the nasals are arched, from an anterior aspect, 
is best shown by an examination of the anterior nares. Their outline varies 
greatly. In some extremes the height, measured from the nasal-premaxillary 
suture, is fully half the width of the opening, while at the other extreme are 
animals with nasals excessively flattened in which the height may be considered as 
being negligible. This latter type occurs in skulls having wide rostra, and a highly 
arched condition of the nares is undoubtedly caused by a medial crowding of the 
premaxillae in early life, due to some obscure stimulus. The degree to which the 
nasals project anteriorly also varies considerably, but in no case do those of adults 
hide the incisive processes of the premaxillae when the skull is viewed from above. 
In juveniles, however, in which the incisors are smaller and project to a lesser 
degree, the latter are barely visible from above. 
MANDIBLE 
It was found that there is considerable variation in mandibular characters. 
The length over all from the condyle to tip of the incisor varies, of course, pre¬ 
cisely as does the glenoid cavity to incisor measurement 
of the skull. The posterior portion of the mandible is 
essentially a system of levers and scaffolding for muscular 
insertions to even a greater extent than is the skull; 
hence its characters are exceedingly sensitive to differ¬ 
ences in the variations of the muscles. These mandibular 
variations, however, are delicate rather than gross, and 
are of a sort difficult to express by measurements. The 
coronoid process varies in length, in width (antero-poste- 
riorly), and in the degree to which it slopes rearward, and 
the same statements apply to the condyle, although varia¬ 
tion in the latter is not correlated with that of the coronoid. There is great differ¬ 
ence, irrespective of age, in the prominence of the slight process formed by the pulp 
capsule of the incisor (the incisive process of the condyle). In some examples this 
is obscure, while in others it is well developed. Its prominence is influenced par¬ 
tially, but not entirely, by the degree to which the condyle is bent inward, that is, 
out of plane with the remainder of the ramus. This, as well as the angle at which 
the whole supero-posterior portion of the ramus projects outward from the supero- 
inferior plane of the molars, varies in degree with the zygomatic width of the 
skull. The widest skulls usually, if not always, are accompanied by the most 
angular mandibles, often with prominent eminences at the roots of the incisors. 
The angular process, as well, usually follows such a trend. Its length, however, 
and its supero-inferior width vary independently of the other two processes, 
development being influenced by different muscles. The length of the angular 
process, measured from the prominence formed by the root of the posterior 
molar to its tip, is given in the tables; but neither this measurement nor that of 
any other portion of the mandible is deemed of great significance in systematic 
work with microtines when the accompanying skull is available. 
The masseteric ridge does not vary individually to an appreciable extent and 
only very slightly with age. The coronoid process and the condyle are pro¬ 
portionately large in juveniles, but the angular process is relatively small. It 
has not been possible to correlate any of these variations save to the extent 
mentioned. Variation in the length of the teeth from one cause or another 
would have a corresponding effect upon the muscles of mastication and their 
insertions, but it is not known with any degree of certainty just what these are. 
96461—24f-3 
Fig. 21 .—Individual variation 
of the nasals and ascending 
branches of the premaxillae 
in adult skulls, a. No. 3830: 
b, No. 3836; c, No. 3871 
