June 7,1924 
Variation in Microtus Montanus Yosemite 
1011 
reentrant angles. In the smaller teeth of juveniles the angles may appear to 
be more acute and sharply defined than in adults, but this seems to be merely 
because of the difference in the size of the teeth and is not relative. There are, 
however, fluctuations in the exact form of the reentrant and salient angles, but 
these are haphazard and purely individual. If they have significance it is 
extremely obscure. 
Any conclusions based upon the length of the teeth must be accepted with the 
greatest caution, for teeth are prone to become loose during cleaning of the skull 
and are then easily pushed in or pulled out beyond their normal positions. After 
drying, it is extremely difficult to detect such misplacement unless it be uneven. 
The length of the anterior molars in relation to the posterior ones, in both 
upper and lower series, is, of course, interdependent upon the length of the 
incisors and the configuration of both the upper and lower diastemata. 
It is entirely permissible to presume that there is some slight fluctuation in 
the rate at which the teeth of different individuals grow from the persistent 
pulp, and there must also be variation in the rate at which the teeth are worn 
away, according to the habits of the individual and the character of the food 
consumed. Longer incisors, brought about by one of these causes, would there¬ 
fore effect longer anterior cheek teeth, for these latter can not wear down more 
rapidly than the wear of the incisors permits. 
Incisors. —In the series at hand there is no relative variation in the incisors 
save in wear and size with age. As the skull increases in size so do the alveoli, 
1 
Fig. 23.—Individual variation in the pattern of the second upper molar M2) : 
a, No. 3825; 6, No. 3815; c, No. 3804 
and consequently the size of the persistent pulp cavities. Hence the incisors of 
adults are of a larger diameter and greater arc of curvature with the projection 
of the free portion correspondingly greater. With increased use of the incisors 
with age there is greater wear upon their posterior faces. The cutting points of 
the lower incisors in adults, however, are practically as sharp (acute) as in imma- 
tures; hence the pits upon the upper incisors are but a shade wider in the older 
animals, often leaving a thin splinter of enamel projecting upon the lateral edge 
of the upper incisors of the larger skulls. 
Maxillary Molars 
First molar. 5 —This tooth is composed of the usual anterior, crescentic loop, 
two inner, and two outer triangles, all closed. There is no appreciable variation 
in its pattern. 
Second molar. —This consists of an anterior crescentic loop, an inner and two 
outer triangles, all closed. Only the posterior space is variable. At one extreme 
is a tendency toward acute sharpness of the posterior enamel faces (fig. 23, a ), 
but this condition is rare, occurring to a marked extent in but two or three 
examples. At the other extreme is a condition in which the posterior triangle 
has developed a slight though definite postero-internal loop (fig. 23, c) or spur 
(metaconule), the enamel space remaining open. This character is well developed 
in but two individuals, both females, but there is a trace of it present in 18 
• The usual terminology of first, second, and third molars is employed. 
