32 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



as this j aw presents ; for the robust incisors may here be regarded as repre- 

 senting molars simplified by the partial loss of enamel, and with a change in their 



direction. 



In the second molar, we find an increase in the antero-posterior diameter, and 



in the length of the tooth, and the enamel at 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 directions 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 corresponding 

 but deeper fold of enamel in the external side of the tooth, we have the grinding 

 surface rendered more complicated by two folds of enamel entering the substance 

 of the tooth from the inner side : these folds divide the antero-posterior extent 

 of the tooth into three nearly equal parts ; they are both directed obliquely for- 

 wards, half-way across the substance of the ivory. 



The^A molar presents the same structure as the fourth, which it exceeds 



only slightly in size. 



In the sixth molar we have a proportionally 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. 



As these grinding teeth by no means increase in the lateral diameter in the 

 same proportion as in their antero-posterior diameter, the posterior ones present, 

 but in a greater degree, the compressed form which characterizes the grinders of 



the upper jaw of the Toxodon. 



It will be seen, however, that there is a difference in the structure of the 

 grinders in this fragment of the lower jaw and those of the upper jaw of the Toxo- 

 don. In the lower grinders there are two folds of enamel proceeding from the 

 inner side of the tooth into its substance, whilst in the upper grinders there is only 

 one fold continued from the inner side ; in the lower grinders there is also a fold of 

 enamel reflected into the substance of the tooth from the outer surface, while in 

 the upper grinders of Toxodon we find the enamel coating on the outer side of the 

 tooth merely bent inwards, so as to describe, in the transverse section, a gently 

 undulating line ; fig. 7, PI. V. is the grinding surface of the sixth molar, right side, 



upper j aw. 



But this difference of structure is by no means incompatible with the co- 

 existence of the two series of teeth in the same animal, since we find the grinders 

 of the upper and lower jaws presenting differences of structure of equal degree in 

 existing herbivorous species. If we examine the jaws of the Horse, for example, 

 we shall find not only an equal amount of difference in the structure of the upper 





