320 COPE: [Vot. IIf. 
strongly indicated by the huge development of these teeth in 
the walrus. This animal uses its canines for the breaking of 
ice, and for lifting itself from the water onto the edge of strong 
ice. The fact that canines and not incisors have been thus 
developed is a necessary result of the fact that the walrus is 
a descendant of a line of animals which had already reduced 
incisors and larger canines. 
Figure 47.— Nimravus gomphodus Cope, two-fifths nat. size; left side of skull; 
from Miocene of Oregon. 
2. DEVELOPMENT OF THE - INCISORS. 
The history of the incisor teeth of the Mammalia exhibits 
three processes; viz.: hypertrophy (e.g. Rodentia), specializa- 
tion (e.g. Galeopithecus, Lemuridz), and atrophy (e.g. Booid- 
ea, Phacochoeris, etc: ). 
Of hypertrophy we have two types: the first represented by 
the Rodentia, Multituberculata, Tillodonta and their ancestors ; 
and second, by the Proboscidia, the narwhal and certain Sirenia. 
As the uses of the incisors present two types corresponding 
with their structure, we have ground for believing the uses in 
question to have been the efficient agent in producing the latter. 
Esthonyx furnishes us with an example (Fig. 48) where all the 
incisors are present in the lower jaw, and where the function of 
one pair of them (the second) has evidently been partially rodent 
