586 UNGULATES. 



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 cement covering the inner side is slightly 

 concave and unbroken. The third molar presents an increase of 

 dimensions in the same directions as the second ; the enamel on the 

 outer side of the tooth presents a similar fold. In the fourth, or first 

 molar, besides a further increase of size and a corresponding but 

 deeper fold of enamel on the external side and nearer the anterior 

 part of the tooth, the grinding surface is rendered more compli- 

 cated by the two folds of enamel entering the substance of the tooth 

 from the distinct plate on the middle of the inner side : these folds 

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

 parts ; they are both directed obliquely forwards, the hinder one 

 goes half-way across the substance of the dentine. The fifth molar 

 presents the same structure as the fourth, which it exceeds only 

 slightly in size. The sixth molar has a much longer antero-posterior 

 diameter, which measures two inches ; but the lateral diameter is 

 but sHghtly augmented ; its structure resembles that of the fifth. The 

 outer coat of enamel extends over half the anterior and posterior 

 ends of the tooth. 



The partial disposition of the enamel upon the molars of the 

 Toxodon is peculiar to that genus ; but the enamel is continued, as 

 in other rootless teeth, to the open end of the implanted base ; it is 

 thinner than in the Rhinoceros. The unenamelled parts of the tooth 

 are coated by a thin layer of cement. The entire body of the tooth is 

 composed of compact dentine, the pulp-fissure which penetrates the 

 middle of the lobes defined by the inflected folds of the enamel, extends 

 from the apex of the open basal pulp-cavity to the grinding surface. 

 The dentinal tubes are J^th of a line at their origin, and radiate 

 an directions vertical to the superficies of the tooth, and of the 

 inflected enamel-folds, and are but little inclined upwards from the 

 ■horizontal plane. They maintain their original diameter, and their 

 relative distance from each other, viz. ith of a fine, to near their 

 peripheral ends. The dentinal cells are sub-hexagonal, and about 

 i,th of an inch in diameter in the peripheral part of the substance. 



