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 
