998 
Journal of Agricultural Research 
Vol. XXVIII, No 10 
of 27.6 and 24.5, a difference of 3.1 mm., or 11.8 per cent of the average. For 
females the average is 25.7 mm., with extremes of 26.9 and 24.6, a difference of 
2.3 mm., or only 8.9 per cent. These figures, as well as subsequent ones, are a 
further indication of the fact that males exhibit greater extremes of development 
with less conservatism of characters than do females. 
Condylo-zygomatic length. —From the condyle to the antero-superior edge 
of the zygomatic process of the maxilla. The tables give an average of 21.8 for 
males, with extremes of 22.9 and 20.3, a difference of 2.6 mm., or 11.9 per cent of 
the average. The average for females is 21.34, with extremes of 22.3 and 20.5, 
a difference of 1.8 mm., or 8.4 per cent. This is an excellent measurement, 
dependable, with little individual variation, and is correlated closely with condylo- 
basilar length. The latter measurement, however, is preferable, and in cases 
where it is taken, as it should be, it is of value when used 
with the condylo-basilar measurement as an index to the 
development of the rostrum. 
Alveolo-basilar length. —The distance from the 
posterior end of the last molar to the posterior edge of 
the incisive alveolus. For males the average of this 
measurement is 15.45mm., with extremes of 16.5and 14.3, 
a difference of 2.2 mm., or 14.2 per cent of the average. 
For females the average is 15.27 mm., with extremes of 
16.5 and 14.6, a difference of 1.9 mm., or 12.4 per cent. 
This is a rather good measurement, as it seems to be of 
considerable importance in general diagnostic work among 
microtines. Its variation is not excessive with age nor 
individually, and it is preferable to the measurement of 
the diastema, to which it is somewhat similar, for the 
latter must be measured with greater care in order to 
prove accurate. 
Condylo-palatal length. —This measurement is from 
the condyle to the anterior point of the bony shelf of the 
palate. The average for this measurement in males is 
12.45 mm., with extremes of 13.3 and 11.2, a difference 
of 2.1 mm., or 16.8 per cent. For females the average is 
12 mm., with extremes of 13.1 and 11.2, a difference of 
1.9 mm., or 15.8 per cent. This measurement varies to a 
considerable extent individually and more with age, but it is probably the most 
valuable measurement to use in precise work as an index to the development of 
this portion of the skull. The chief and probably the only objection to its use 
is the fact that while taking it the delicate processes often present upon the bony 
shelf of the palate, which may be of the greatest importance in specific or group 
diagnoses, are extremely subject to injury. 
Height of skull. —The perpendicular distance from a plane passing from 
the most inferior point of the bulla along the crown of the most prominent molar 
to the highest point of the cranium. The average of this measurement for 
males is 10.72 mm., with extremes of 11.2 and 10.2, a difference of 1 mm., or 
9.3 per cent. For females the average is 10.65 mm., with extremes of 11.1 and 
10.1, again a difference of 1 mm., or 9.4 per cent. The height of the cranium 
is an extremely useful measurement to take, especially when it is used in com¬ 
parative work with the condylo-basilar length as an index. Another character 
that is valuable in the case of micro tines is the angle formed by the molar row 
with the plane passing from the bullae to the most prominent molars. In many 
Fig. 13.—Variation of the 
skull with age in Microtus 
m.yosemite: ventral aspect. 
Left half of the skull of 
adult No. 3855, upon the 
left, separated by medial 
line from right half of skull 
of juvenal No. 3857, upon 
the right: condylo-basilar 
dimensions of both mag¬ 
nified to the same size 
