606 HISTORICAL GEOLOGY 



did rhinoceroses that are ancestral to existing genera live in North 

 America, but a number of side branches were also developed on that 

 continent which differed widely from those of to-day, some of which 

 lived, at least locally, in great abundance. In a remarkable deposit 

 of the Lower Miocene (Harrison Beds, near Agate, Nebraska) a 

 slab of rock 10 feet by 40 feet by 18 inches was uncovered by an 

 American Museum party in 191 2, in which are 75 skulls of a species 

 of rhinoceros (Diceratherium), together with the bones of these and 

 other mammals. This deposit is without doubt exceptional, but 

 nevertheless shows that, in certain localities, these rhinoceroses were 

 extremely abundant. In the Oligocene of North America three 

 branches of the family are known, but in the Miocene they had 

 evolved into a number of branches, which, however, may be united 

 into three groups. 



(1) One of these may, for convenience, be called the " swimming 

 rhinoceros " because of the spreading, four-toed foot which was 

 doubtless an efficient organ for swimming. This branch (Metamyno- 

 don) was apparently semi-aquatic and was fitted for life in the lakes 

 and rivers of the Oligocene. It was stout and rhinoceros-like in 

 shape, the eyes were placed high on the head, and the nostrils opened 



upward so that it could breathe 

 when the head was partly sub- 

 merged. Its canine teeth were 

 elongated into tusks and were 

 doubtless used for uprooting the 

 plants from the bottom and banks 

 of the lakes and rivers which it 

 frequented. 



Fig. 545. — A running rhinoceros (2) A second 'Oligocene rhinoc- 

 (//yr^orfon) showing the modification of eros hr ^ nch whose ca jj ke 



structure for plains conditions. (After . . 



Prof. H. F. Osbom.) that of the swimming rhinoceros, 



terminated before the close of that 

 epoch, was the " running rhinoceros " (Hyracodon). This animal 

 (Fig. 545) did not have the appearance which is usually associated 

 with the rhinoceros, since it was light-limbed and agile, with 

 horselike shoulders and limbs. It had three toes on each foot, 

 very similar to those of the horse of its time, and was apparently 

 adapted to the hard, dry plains of the Oligocene. It is possible 

 that, had this animal succeeded in adapting itself to the changing 

 conditions of the Tertiary and in competing with the other grazing 



