No. 2. ] THE HARD PARTS OF THE MAMMALIA. 243 
If this homology is correct, the Dinocerata were derived 
directly from the Pantolambdidz, and not through the Corypho- 
dontidz. 
The mechanical causes of the peculiarities of the Dinocera- 
tous dentitions are then the following : — 
First, development of anterior cingular crest ; cause same as 
in Coryphodontide. 
Second, loss of anterior limb of anterior external V of superior 
molars ; cause, disuse. 
Third, shearing of oblique crest of inferior molar in front of 
instead of behind posterior limb of anterior external V of supe- 
rior molar. Cause, development of anterior basal cingulum of 
superior molar, which wedges cross-crests of inferior molar ante- 
riorly. 
Figure 67, — Uintatherium, mandible anterior to coronoid process, one-fourth nat. 
size; from Bridger beds of Wyoming. From Osborn, memoir on Loxolophodon and 
Uintatherium. 
Fourth, loss of posterior cross-crests of inferior molars. The 
answer to this question is the answer to the other question, 
Why was the oblique crest of the inferior molar developed in 
the Uintatheriidz while it remained rudimental in the Corypho- 
dontidz ? The answer to these questions is the explanation of 
the principal peculiarities of the former family. The answer 
appears to me to be simply that while the movement of the 
lower jaw in mastication was probably ectal in the Coryphodon- 
tide, it was probably ental in Uintatheriide. This explanation 
is largely hypothetical, yet is accords with the relations between 
use and the development of the crests in the two families. In 
the ectal movement in Pantolambda the oblique crests of the 
opposing molars are soon separated from mutual contact, so 
that none of them have use on the internal half of the crown 
