48 W. H.’F. ADDISON AND J. L. APPLETON, JR. 
cisor is in one plane only, the lower incisor is a portion of a flat- 
tened spiral, possessing a curve in three planes. The upper 
incisor is a segment of a true circle (at 5 months about 210°) 
and in cases of overgrowth it has often been known to complete 
the circle. In the case of the lower incisor, however, when 
we project it on the sagittal, frontal or coronal planes, it gives 
in each case a curve. It was the very evident curved projec- 
tion seen on the sagittal plane to which Owen referred. Con- 
sidering only this view, the lower incisor of a 5-month animal 
forms a segment of about four-fifths of a semicircle (140-145°). 
TABLE 1 
23 41 10 15 5 5 10 
DAYS | DAYS WEEKS WEEKS MONTHS MONTHS MONTHS 
mm. mm. mm. mm. mm. mim. mm. 
Naso-occipital length.............. 297 | 32.5 | 39 40 43 44 46.5 
Do) LAS Galea sale Gy elope 
~J 
= 
— 
= 
Ts 
o 
Inierzy gomatiey. 2.0.0 .c/2-) see ilse 
Upper diastema. . eel alerts Oo sone LO A ZED aie 
Upper Ee ee it iene PR oe IRS LS 1823 | 2023) | 23.35) 237 | 2022 
Upper incisor—extra-alveolar 
lenothy 2.0: ere anche eae Byell || Bese ip “7 8.4: 8.7 | 9 93 
Lower diastema. . i rE et On| D0) |) 16 Oat || 7 6.8 
Lower saeiao “tata ence Ee aoe 18.1 | 20.7 | 25.5 | 26.4.4 29.41.29 9iotes 
Lower incisor—extra-alveolar 
lento thie fava is. cite Sask eee aera @5).| - 7 WO)ecaye |) TEL aah ete) ee 12.4 
Measurements of the incisors and skulls of animals of differ- 
ent ages, were made as shown in table 1. 
The teeth were measured along their convex surfaces by 
means of silk thread wet with water, and applied to the object 
to be measured. The thread was then cut with scissors at the 
end of the object, straightened on paper and measured to tenths 
of millimeters. 
A consideration of table 1, shows in a definite way the pecul- 
iarities characteristic of the dentition, not only of the rat but 
of rodents in general. As is well known, these are the great 
development of the incisors, the wide diastema, and the con- 
sequent posterior position of the molar teeth as related to the 
rest of the skull. Cope (’88) wrote that he considered “the 
