No.2.] ZHE HARD PARTS OF THE MAMMALIA. 221 
in character; that is, it has served as a scraper and gouger of 
food substances. Persistent use has apparently developed the 
size of this pair of teeth, until we find in Psittacotherium (Fig. 
49), they have reached = 
a greater efficiency, 
and that the external 
incisors of the lower 
jaw have disappeared. 
This disappearance 
can be accounted for 
on the ground of dis- 
use, a retirement from 
service due to posi- 
tion, and the ain- 
creased growth of 
mcison INo, 2.° In S&S we 
Calamodon (Fig. 85) 
the first incisor has 
become rudimentary 
from the same cause, 
and in Anchippodus 
STA ™Wy-u 
SSS 
Figure 48. — Esthonyx burmeisteri Cope, denti- 
tion: a, profile; 4,superior; ¢, inferior dentition, grind- 
it has disappeared al-_ ing faces. Reduced. 
together, leaving a 
truly rodent incisor 
dentition, consisting 
of the second incis- 
ors only, in the lower 
jaw. Continued use 
as chisels has devel- 
oped these teeth to 
the great proportions 
seen in such Rodentia 
ASHE AStOroides, 1 cic 
(Fig. 86). 
The use which the Figure 49.— Psittacotherium multifraguin Cope, 
Proboscidia and _ Si- mandibular ramus, one-half nat. size: @, profile; 4, 
from above. 
renia (Halicore) give 
their incisors, is, from a mechanical point of view, like that 
which the Carnivora give their canines; that is, it consists of 
strains transverse to the long axis of the tooth. The elephants 
