52 W. H. F. ADDISON AND J. L. APPLETON, JR. 
Ryder (’77) suggested a classification of rodents based on the 
shape of their incisors as seen in cross-section. In some genera 
the diameter of the teeth is less from side to side, than in the 
antero-posterior direction, while in others the reverse condition 
is found. The present form belongs to the former group, as 
is shown in figure 4. From the consideration of many rodents, 
Ryder deduced the general principle, that where the incisors 
are thicker in the antero-posterior direction, the gnawing habit 
is greatly developed. 
MINUTE DESCRIPTION OF THE INCISORS 
Enamel and dentine make up the hard tooth substance, en- 
closing the pulp. Owen, in his ‘‘Odontography” (’40—’45, p. 
399) said that there existed a general investment of cementum 
over the whole tooth structure. J. Tomes (50, p. 533) was not 
able to agree entirely but said that in most, if not in all, incisors 
of rodents cementum could be seen investing the posterior sur- 
face. In the rat, it is not apparent that there is any cementum 
at all. The enamel is usually colored with a pigment which 
is yellowish in the young but becomes orange-colored with age, 
and is usually more pronounced in the upper than in the lower 
incisors. At 13 days, there is as yet no color, but at 21 daysa 
slight tinge of yellow is perceptible in the uppers, but none in 
the lowers. At 25 days the uppers are distinctly yellow, and 
the lowers have now acquired a slight color. At 38 days, these 
colors have intensified, the uppers having more pigment than 
the lowers; and in the mature animal the same relation con- 
tinues, the uppers being orange-colored and the lowers yellow. 
The enamel is found principally on the labial side, and this 
accounts for the shape of the occlusal surface. For, the enamel 
being harder than the dentine, the latter is more easily worn 
away by the action of the opposing tooth, and the more resistant 
enamel remains as the cutting edge or point. The shape of the 
incisal end of the upper and lower teeth is different, being chisel- 
like (sealpriform) in the upper, and more rounded and narrower 
in the lower. The incisal line is also usually different in the 
