222 COPE. VoL wii: 
use their tusks for prying up the vegetables on which they feed, 
or for pushing aside the vegetation through which they wish to 
pass. The ancestors of the Proboscidia are not certainly known, 
but they possessed incisors of enlarged proportions, such as we 
find in the Toxodontia and other late representatives of some of 
the primitive Ungulata. Use of such teeth in the manner re- 
ferred to, without opposition from the inferior incisors, will 
account for the tremendous proportions which they ultimately 
reached in some of the species of Elephas. 
The use made by the narwhal of its single huge superior 
incisor, that of an ice-breaker, indicates the origin of its large 
dimensions, and its primitive limited use in proportion to its 
small beginnings. So with the straight incisors of the hippo- 
potamus ; use as diggers has straightened them to a horizontal 
from their primitive vertical direction, a change partially accom- 
plished in the true pigs (Sus.) 
Figure 50.— Lemur collaris, dentition from below and aboye; nat. size; original. 
In the Sirenian genus Halicore the upper incisors have been 
used in excavating vegetable growths from the banks and bottom 
of shallow seas. The transition from three incisors (Prorasto- 
mus) to two (Dioplotherium), and to one (Halicore), is identical 
with what has taken place in the Proboscidia and Rodentia, and 
has resulted in the production of an effective digging tool. In 
other genera, whose habits of browsing on soft growing mate- 
rials did not necessitate the use of digging incisors, these teeth 
became atrophied, as in the manatee and Rhytina. 
The inferior incisors of the true lemurs are straight, horizontal, 
