34 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



In speculating upon the nature of the organized substances which the teeth 

 of the Toxodon were destined to grind down, we must not only take the structure 

 of the tooth into consideration, but also the power of perpetual renovation, which 

 will compensate for the defective quantity of enamel in the grinders of the 

 Toxodon, as compared with those of the existing Ruminants and Pachyderms, 

 whose grinders, when once completed, receive no further addition of dental sub- 

 stance at their base. The Toxodon, in this character of its dentition, participated 

 in the same advantages with the Capybara and the Megatherium. 



Although we have been enabled to observe the structure of the grinding teeth 

 of the upper jaw of the Toxodon in two examples only; one, an insulated perfect 

 grinder corresponding to the sixth alveolus on the right side, and the other, a 

 portion of the last grinder of the left side remaining in the socket of the head 

 previously described, yet from the relations subsisting between socket and tooth, 

 a very satisfactory opinion may be formed of the structure of those teeth which are 

 wanting, as well as of their size. It thus appears, that the grinders of the upper 

 jaw of the Toxodon, are small and simple at the anterior part of the jaw, and that 

 they increase (chiefly in antero-posterior extent) in size, as well as in complexity, 

 as they recede backwards in the jaw. In this respect, as well as in size, the teeth, 

 in the fragments of the lower jaw just described, exactly correspond. There is, 

 however, a slight difference in the lateral diameter of the two sets of grinders, 

 those of the lower jaw being narrower, as is usually the case, but not in the same 

 degree as in the Horse or Ruminant. A greater difference obtains in the degree of 

 curvature of the two sets of molars, those of the lower jaw, especially the posterior 

 grinders, being much less bent than the corresponding teeth of the upper jaw. 

 It is necessary to observe, also, that the convexity of the curve of the inferior 

 grinders is directed outwards, as in the superior grinders ; while in the Guinea Pig 

 and Wombat, which have also curved grinders, the convexity is outwards in the 



lower jaw, and inwards in the upper jaw. 



Nevertheless, if we take into consideration the close similarity which exists 

 between the teeth of the upper jaw of the Toxodon, and those of this lower jaw in 

 more essential points, as in their persistent pulps, their characteristic structure and 

 form, the depth of their sockets, and their relative sizes and complexity ; and when 

 we consider how the depth of this lower jaw, and its narrowness in the transverse 

 direction, corresponds with the characteristic form of the upper jaw of the Toxodon, 

 and that to these resemblances is added an apparatus of incisors adequate to 

 oppose the great dentes scalprarii of the upper jaw, the conclusion seems irresisti- 



of temporary growth, is, of necessity, dependent on the duration of these essential aids to nutrition ; thus, a 

 sheep generally wears down its grinders in twelve years, and its natural term of life is consequently limited to 

 about that period. 



