tHE ANCIENT MAMMALS OF BRITAIN. 317 



Crag assists in explaining the present anomalous distribution of 

 these animals ; one species of which is Malayan, while all the 

 rest are South American. Although the Straight-tusked Ele- 

 phant occurs in the Norwich Crag, while the southern Elephant 

 dates from the subjacent Red Crag, the commonest Probosci- 

 deans of the period under consideration were Mastodons, 

 which, it is scarcely necessary to mention, differ from Ele- 

 phants in the much lower crowns of their molar teeth, which 

 are surmounted by low tubercles, frequently arranged in a 

 small number of transverse ridges, separated from one another 

 by more or less completely open valleys, this type of tooth 

 being much more generalised than that of the Elephants. In 

 one of the Crag Mastodons {M. arvernensis) the tubercles of 

 the molars were arranged alternately, and the lower jaw was 

 short and devoid of tusks ; in a second {M. longirostris) the 

 same tubercles were arranged in transverse ridges, with their 

 worn summits showing a trefoil pattern like those of the Hip- 

 popotamus, the lower jaw beirig at the same time greatly pro- 

 duced and armed with a large pair of tusks j while in the 

 third {M. borsont) the ridges in most of the teeth were three, 

 instead of four, m number, and retained much less distinct 

 evidence of their constituent tubercles. 



The Rodents need not detain us long, but the Giant Beaver 

 of the Forest-bed was sparingly represented in the Norwich 

 Crag, while a smaller member of the same gtriMS {Trogontherium 

 minus) is found in the subjacent beds ; the only other named 

 Rodent being the extinct Vole referred to above, which ranges 

 downwards from the Forest-bed to the Norwich Crag. 



The Whales and Dolphins of the Crag are so interesting that 

 they would afford ample material for an article by themselves, 

 and can, therefore, receive but scant notice in the limits of 

 space available. Among the Whale-bone group there appear to 

 have been no less than four species of Right Whales, one of 



