131 



side, the Toxodon agreeing in this respect with most other large herbi- 

 vorous Mammalia, the fixed surface for attrition in the upper jaw being 

 from obvious principles more extensive than the opposed moveable sur- 

 face in the lower jaw. 



The first grinder in the lower jaw is of small size and simple structure, 

 being surrounded with a coating of enamel of uniform thickness and 

 without any fold penetrating the substance of the tooth. It is more 

 curved than any of the other molars, and appears to have differed from 

 the external incisor only in its entire coating of enamel and direction of 

 growth : it is interesting, indeed, to find so gradual a transition, in struc- 

 ture, from molar to incisive teeth as this jaw presents ; for the robust 

 incisors may here be regarded as representing molars simplified by the 

 partial deficiency of enamel, and with a change in their direction. 



The second molar presents an increase in antero-posterior diameter, 

 and in length, and the enamel of the middle of the outer side makes a 

 fold which penetrates a little way into the tooth ; the line of enamel on 

 the inner side is slightly concave and unbroken. 



The third molar presents an increase of dimensions in the same direc- 

 tions as the second ; the enamel on the outer side of the tooth presents 

 a similar fold, but it is directed a little more backwards. 



In the fourth molar, besides a further increase of size and a corre- 

 sponding but deeper fold of enamel on the external side of the tooth, 

 the grinding surface is rendered more complicated by two folds of enamel 

 entering the substance of the tooth from the inner side : these folds di- 

 vide the antero-posterior extent of the tooth into three nearly equal 

 parts ; they are both directed obliquely forwards, half-way across the sub- 

 stance of the dentine. 



The fifth molar presents the same structure as the fourth, which it ex- 

 ceeds only slightly in size. 



The sixth molar presents a proportionately greater increase of size 

 in the antero-posterior diameter, which measures two inches ; but the 

 lateral diameter is but slightly augmented ; its structure resembles that 

 of the fifth. 



The outer coat of enamel is interrupted for a brief space at the ante- 



s 2 



