SOME EXTINCT ARGENTINE MAMMALS 85 



regular even series without gap or interval, and with 

 their crowns of equal height. Very different in dental 

 character are the members of the allied genus Astra- 

 potherium, in which each jaw was furnished with a huge 

 pair of tusks, those of the lower jaw curving outwards 

 and upwards after the manner of those of a wild boar, 

 while both were kept sharp and keen by their points 

 wearing against one another. In the presence of these 

 enormous upper tusks, the astrapotheres resembled the 

 extinct uintatheres of North America, although they differed 

 in the possession of tusks in the lower jaw, while it is 

 probable that those of the upper jaw were incisors instead 

 of canines. One of the most curious features connected 

 with these animals is the close resemblance of their upper 

 cheek-teeth to those of rhinoceroses, the similarity being 

 so marked that if we were acquainted with the South 

 American animal only by these teeth, it would probably 

 be classed with the rhinoceroses. From the structure of 

 the bones of the ankle it is, however, quite certain that 

 these two groups of ungulates have no direct connection 

 with one another, and that their common ancestor had 

 teeth of a much simpler type of structure. It follows, 

 therefore, that the form of cheek-teeth characterising both 

 the astrapotheres and the rhinoceroses has been evolved 

 independently in the two groups, and that we have con- 

 sequently here another case of parallelism. Although this 

 type of tooth (which, it must be remembered, is one of 

 considerable complexity) is admirably adapted for crushing 

 vegetable substances, it is by no means the only one which 

 could have been evolved from what we may probably regard 

 as the primitive type, and it is therefore difficult to see how 

 it can have been produced by evolution unaccompanied 

 by design. 



