June 7, 1924 
Variation in Microtus Montanus Yosemite 
995 
basilar length is 115.2 pellets, while that of the eight smallest, normal subadults is 
110.3, or 95.6 per cent. A similar comparison of the condylo-basilar length of 
these individuals gives 91.6 per cent. Comparison of the braincases of the 
largest superadult and smallest juvenile male results in figures that are mislead¬ 
ing (73.8 per cent), for the reason that the brain capacity of the former is abnor¬ 
mally large. Comparison of the aver¬ 
age of the three largest specimens with 
that of the three smallest, however, gives 
83.4 per cent, while a similar treatment 
of their condylo-basilar lengths results 
in a percentage of 84.5. It is perhaps 
wiser to ignore the fact that the two lat¬ 
ter percentages occur in reverse order to 
those first stated, for as the difference 
is small this is very likely due to indi¬ 
vidual variation. As this latter is consid¬ 
erable, the reader must judge for himself 
whether the figures indicate true condi¬ 
tions, and whether or not to consider the 
braincases of the younger animals pro¬ 
portionately larger than those of the 
older ones. At any rate, this difference 
with age is assuredly much less than has 
been popularly supposed. In skulls with 
the largest braincases the increased capac¬ 
ity is usually due to a slight proportional 
increase in the length, rather than in the 
breadth, of the cranium. 
Aside from such facts as are offered 
above, the external surface of the brain- 
case has certain details that do make it 
appear definitely of greater relative size 
in comparison with the whole skull in 
the case of the juvenile. While the con- 
Fig. 10.—Diagrammatic lateral aspect of adult 
skull, a, Anterior nares; &, nasal; c, anteorbital 
fossa; d, lachrymal; e, zygomatic process of 
maxilla; /, frontal; g , orbitosphenoid; h, sphe¬ 
noidal fissure; i, squamosal; j, external ptery¬ 
goid plate; fc, parietal; l, prelambdoidal fenes- 
trum; m , interparietal; n , lambdoidal crest; 
o, occipital; p, incisive process of premaxilla; 
q, premaxilla; r, anteorbital foramen; s, max¬ 
illa; t, optic foramen; u, jugal; v, alisphenoid; 
w, foramen rotundum; x , foramen ovale; y, 
internal pterygoid plate; z, premastoid vacuity; 
a', mastoid; external auditory meatus; c' 
audital bulla; d' t mastoid; e f , paroccipital pro¬ 
cess. 
Fig. 1L—Diagrammatic posterior aspect of adult 
skull, a, Lambdoidal process; 6, exoccipital; c, con¬ 
dyle; d, basioccipital; e, supraoccipital; /, mastoid; 
g, foramen magnum; h, paroccipital process; t, 
audital bulla. 
dylo-basilar length of the smallest fe¬ 
male is 80.8 per cent that of the largest 
the distance between the prelambdoi¬ 
dal fenestrations of the squamosals is 
actually 0.4 mm. greater in the juve¬ 
nile, and the transverse measurement taken between the posterior termination 
of the postorbital processes is 92 per cent that of the adult. The proportional 
width (of braincase only) at the zygomatic processes of the squamosals is also 
about 92 per cent; but this is a very difficult measurement to take with accuracy 
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