38 THE AMERICAN BEAVER. 



upon Fischer's description. Owen afterward, by means 

 of additional specimens, detected variations in the 

 forms of the jaws and teeth which led him to question 

 this classification, and to assert a sub-generic position 

 for this animal. He remarks: "The well-marked dif- 

 ferences which the English fossils have demonstrated, 

 not only in the proportions, but in the form and struc- 

 ture of the teeth of the Trogontherium, will, I trust, 

 be allowed to yield the same grounds for its sub-gen- 

 eric distinction as has been proposed or accepted by 

 the best modern zoologists for the subdivisions of the 

 same value in the rest of the rodent order."^ Tlie 

 Trogontlierium was about one-fifth larger than the Eu- 

 ropean beaver, the skull measuring seven inches and 

 three lines from the occipital ridge to the most convex 

 part of the incisors. 



Since both the European beaver and the Trogon- 

 therium have been found in a fossilized state in the 

 newer pliocene formations, and in deposits which 

 have yielded remains not only of the mammoth and 

 the rhinoceros, but also of the mastodon, and since 

 there is evidence tending to show that the American 

 beaver was cotemporaneous with the mastodon, the 

 generic type of Castor, and also the family type of 

 Castoridx are thus carried far back into the tertiary 

 period. 



Upon the American Continent the American bea- 



modification since the period of the mastodon. This animal, and 

 the fossil elephant, Elephas primigeneus, were coeval with the 

 existing flora and the present conditions of the surface of the con- 

 tinent; and there are no reasons, geologically, why they may not 

 have coexisted with the human race." 



^ British Fossil Mammals and Birds, p. 188. 



