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 
independentiy 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. 
