June 7,1924 
Variation in Microtus Montanus Yosemite 
1005 
Fig. 20—Individual variation in the palatal shelf and internal 
pterygoid plates of the adult skull, a, No. 3812; 6, No. 3845; 
c, No. 3837; d, No. 3820 
elongation of the external pterygoids to form an angle with the axis of the skull 
of increasing acuteness, this in response to the development of the pterygoid 
muscles, chiefly the internal ones. It should be noted that there occurs a mini¬ 
mum of variation in the size and depth of the pterygoid fossae, except with age 
as noted. 
Between the maxilla, orbitosphenoids, and alisphenoids occurs the large sphe¬ 
noidal fissure, through which pass several important nerves and blood vessels. 
Directly supero-lateral to the external pterygoid plate are situated the foramen 
rotundum and foramen ovale, the latter being the posterior one and by far the 
larger. In this, however, as in many other species, the foramen rotundum varies 
much in size, and in a few cases is virtually blended with its neighbor. 
Basisphenoid. —This bone articulates posteriorly with the basioccipital and 
anteriorly with the presphenoid. It then descends antero-inferiorly, articulating 
with the palatines to form 
all but the anterior base of 
the internal pterygoid plates. 
The numerous foramina and 
vacuities in the pterygoid 
region are large and extensive 
in the present species. Espe¬ 
cially worthy of note is the 
vacuity in the anterior portion of either internal pterygoid plate and those on 
either side of the presphenoid. At times these will be found to be of aid in diag¬ 
nosis. On the whole, these vacuities are rather constant individually, but they are 
proportionately very much smaller in younger animals. The internal pterygoid 
plates not only appear to be relatively heavier in juveniles because shorter, but 
they are actually heavier (wider) in inferior aspect than in the majority of older 
examples. There is also considerable individual variation not only in the ap¬ 
parent thickness of these plates, but in the shape. In some cases the two are 
practically parallel, in the majority they diverge slightly, while in the case of a 
few there is a marked tendency for the posterior halves to flare. 
Presphenoid. —In the present species, as in many, if not most, microtines, 
the juncture of the basisphenoids and presphenoids can not be distinguished 
with certainty, even in juveniles. It can be described in the genus Ondatra, 
however, and from analogy with that it is possible to state with a degree of cer¬ 
tainty that this bone begins at a point slightly posterior to the posterior vacuities 
upon the floor (or rather roof) of the interpterygoid fossa. It thence extends 
anteriorly, in the form of a rod, to join the vomer. The slender bridges of bone 
joining it upon either side are the presphenoid processes of the basisphenoid. 
Orbitosphenoids. —Externally the orbitosphenoid is of little consequence 
in the present species. It appears as a very small bone forming the antero¬ 
medial wall of the sphenoidal fissure and is pierced by the optic foramen. It 
articulates with the alisphenoid, maxilla, and frontal. 
Palatals. —The suture between the paired palatals can be distinguished only 
in the youngest juveniles at hand. Each of these bones constitutes the extreme 
anterior portion of the internal pterygoid plate, the anterior part of the pterygoid 
fossa, the anterior half of the medial face of the external pterygoid plate, and 
together, the palatal shelf. The palato-maxillary suture lies parallel and just 
medial to the molar alveoli, and the anterior border of the bone is situated at a 
point lying between the middles of the second molars. Important details to 
be considered are the shelf of the palate and the palatal pits. There is much 
variation in the width and shape of the former, this being of a nature impossible 
