THE EVOLUTION OF THE MAMMALIA. 225 
mainly or exclusively, in their distribution. The most ancient 
genera which can be definitely referred to this series are found in 
the White River beds and in the contemporary formation of 
Ronzon, in France, though certain fragmentary fossils of the 
Uinta, Bridger, and even the Wasatch, when better known, will 
not improbably turn out to be still more primitive members of the 
same line. Until this point can be cleared up, the place of origin 
of this series must remain uncertain. 
The typical White River rhinoceros (Canopus) was an animal 
of moderate size, which is rather lightly built, having proportions 
not unlike those of the tapir, and the bones are in very marked 
contrast to the massive skeleton of the existing rhinoceroses. 
The skull is already of distinctly rhinocerotic type, but the 
absence of any horn, the slender and pointed nasal bones, 
and the straight upper profile of the head, combine to give it 
quite a different appearance from that of the living species. The 
anterior teeth are more numerous than in the latter, but they have 
already taken on the characteristic shape and function, as have 
also the grinding teeth ; but these are of somewhat simpler pattern 
than in the modern genera, and not all of the premolars are yet like 
the molars in construction. The neck is short and heavy, and 
the trunk is long, and is of by no means such excessive girth as in 
the rhinoceroses now living. The limbs also are proportionately 
slender, and while they are of no great length, yet they are decidedly 
more elongate than in the recent types. This is especially true 
of the feet, which are longer and narrower than in the latter, and are 
already reduced to three digits each, the minimum number which 
occurs in any member of the family. In short, in almost every 
point of dental and skeletal structure, Ccsnopus is much more 
primitive and less specialized than the subsequent members of 
the series. The French type, found at Ronzon, is very much 
less completely known than the contemporary American forms, 
but would seem to have been very like them. 
As we ascend through the successive strata of the White River 
formation, species of Ccsnopus are found at all levels, and when 
compared with one another, are seen to display gradual and 
