SOME EXTINCT ARGENTINE MAMMALS 83 



teeth grow for a considerable portion of life, yet they 

 eventually form roots in the ordinary manner ; the same 

 being true of the incisors, with the exception of a single 

 pair, which grow permanently. We see, therefore, that the 

 permanently growing teeth of the toxodon are a specialised 

 feature, and the older genus shows that these animals 

 are clearly allied to the odd-toed ungulates, although 

 sharply distinguished by the structure of the feet. Indeed, 

 since their feet are of a more generalised type than those 

 of the latter (as is especially shown by the almost flat 

 huckle-bone), while their teeth are more specialised, it is 

 evident that neither group can be ancestral to the other. 

 Hence the toxodon and its allies may be regarded as 

 forming a separate group of equal value with the other 

 subdivisions of the great ungulate order. When these re- 

 markable creatures branched off from the primitive ancestral 

 types of the latter, and how they first obtained an entrance 

 into South America, where they gradually increased in 

 size and specialisation till the period of the Pampean, when 

 they finally disappeared, are still unsolved problems. 



The interest of the toxodons does not, however, by any 

 means end here. Although, as we have seen, the toxodon 

 itself shows certain resemblances to rodents in the structure 

 of its teeth, it will be evident that such resemblances indi- 

 cate no genetic affinity between the two groups, since 

 rodents are neither the ancestors nor the descendants of 

 the toxodons. In a much smaller animal, known as the 

 typotherium, these rodent resemblances are still more 

 pronounced, as is especially shown by the incisor-teeth, 

 which are essentially those of a rodent. Moreover, in the 

 hind-feet the toes have lost the hoofs characterising the 

 more typical ungulates, and were probably protected by 

 small nails. A still further step is exhibited by a much 



