June 7,1924 
Variation in Microtus Montanus Yosemite 
1007 
The anterior portion of the braincase, and therefore the posterior portion of 
the interorbital, is amenable to the effects of the anterior part of. the temporal 
muscles. No important muscles of mastication arise from the anterior portion 
of the interorbital, such as would logically develop a crest or ridges, and other 
causes are therefore responsible for their formation. Explanatory theories may 
be suggested for the growth of such, but these can not be proved. 
Lachrymals. —It was not discovered until after all skulls had been cleaned 
that in their preparation by means of a fine stream of water, the lachrymals 
had become loosened and washed away from all but a very few. This bone, 
however, evidently varies to only a slight extent. When the skull is viewed 
from above, the lachrymal is seen as a small bone upon the antero-supero-medial 
border of the orbit. It embraces a small portion of the frontal and then ex¬ 
tends along the zygomatic root of the maxilla to slightly lateral of the small 
lachrymal process of this bone. There is also a thin plate extending anteriorly 
along the supero-medial face of the anteorbital foramen quite to the anterior 
opening. 
Jugals. —The jugal or malar articulates posteriorly with the corresponding 
process of the maxilla. There is some variation in this bone, such depending 
upon differences in the zygoma as a whole. The posterior suture is strong, 
the jugal fitting into an indentation in the process of the squamosal, dovetail 
fashion. The anterior suture is discussed under the next heading. 
Maxillae. —Each of the two maxillae articulates with the frontal, premaxilla, 
lachrymal, jugal, orbitosphenoid, alisphenoid, and the palatine. The suture 
with the frontal occurs at the superior base of the zygomatic process, this uniting 
with the maxillo-premaxillary suture, which descends to join the incisive fora¬ 
men at a point slightly anterior to its center. The junction between the two 
maxillae, extending from these foramina posteriorly to the palatines, is obliter¬ 
ated, even in the youngest animals at hand. Skirting the lateral edge of the 
palatine, the entire maxillary molar series on either side is included within the 
maxilla, which then extends from just posterior to the third molar antero- 
superiorly to the sphenoidal fissure. Thence the frontal-maxillary suture 
ascends anteriorly to about the center of the anteorbital foramen. 
The suture with the frontal at the superior base of the zygomatic process is 
usually practically parallel with the axis of the skull, but it may slope antero- 
medially at a considerable angle in some cases. There is practically no varia¬ 
tion in the remaining sutures. As previously indicated, there is great variation 
in the development of the zygomatic process of the maxilla. In juveniles this 
is weak, and it may be almost as lacking in strength proportionately in sub- 
adults. Robustness in this detail is correlated with strength in other portions 
of the skull and is the result of stimulation by masseter muscles that are stronger 
than the average. There is little difference* save in the degree of lateral spread, 
between an angular and a weak skull when viewed from in front. When exam¬ 
ined from above, the zygomatic processes of the former will be seen to project 
from the skull more nearly at right angles. The slight indentation in the bone 
immediately antero-superior to the anteorbital foramen is always well defined in 
angular skulls, which may or may not be the case in weaker ones. The slight, 
spinous process upon the supero-postero-medial border of the root—the lachry¬ 
mal process—is even more irregular, its precise development, or absence, depend¬ 
ing upon the adjoining lachrymal. The exact position of the anteriormost 
point of the maxillary-jugal suture is quite variable. In some specimens it is 
situated about even with the anterior face of the first molar, but in the majority 
it is about one millimeter, and in a few a trifle more to the rearward. The 
jugal is slightly dovetailed into the maxilla, and the suture extends posteriorly 
