468 LAND MAMMALS IN THE "WESTERN HEMISPHERE 



much slighter and more slender build than ]Toxodon, though 

 every tooth and every bone proclaims its relationship to the 

 latter. 



In ]Nesodon the dental formula was unreduced; i|, c\, 

 P 4) w* I) X 2 = 44, though several of the teeth were much 

 reduced in size, so as to have lost their functional impor- 

 tance, and frequently individuals are found in which one or more 

 of these insignificant teeth are lacking. The first upper incisor 

 was a broad, chisel-shaped tooth, which continued to grow for 

 a period, then formed its root, and growth ceased ; the second 

 incisor was a pointed, triangular tusk, which grew throughout 

 life, becoming longer with advancing age ; while the third, which 

 was lost in ^Toxodon, was small and unimportant. In the 

 lower jaw the first and second incisors were chisel-like and had 

 a limited growth ; being rather narrow, they both bit against 

 the broad first upper incisor ; the third incisor was a persist- 

 ently growing tusk, not so large as the upper one, against the 

 posterior face of which it impinged and was obliquely trun- 

 cated by wear, so that its length was Umited, while the upper 

 tusk continued to elongate and was made narrower and sharper 

 by wear. All the lower incisors were far less procumbent than 

 in ^Toxodon, and were directed obliquely upward and forward. 

 The remarkable changes of appearance which, as mentioned 

 above, took place within the life-time of the individual, were 

 largely due to the differential growth of the incisors. The 

 milk-incisors were all nearly alike and formed no tusks ; when 

 the permanent incisors were first protruded, the first upper 

 and the first and second lower were large and the tusks were 

 not visible, and, when the latter did appear, they were for some 

 time smaller than the other incisors. These, however, formed 

 roots and ceased to grow, actually becoming smaller with 

 advancing age, for the crowns narrowed to the roots and, the 

 more they were worn down, the smaller they became. The 

 tusks, on the other hand, grew throughout life and became 

 larger as the other incisors were reduced by wear, and thus the 



