Cope.] 88 [Feb. 17, 
The mechanical causes of the peculiarities of the Dinoceratous denti- 
tions are then the following :— 
First, development of anterior cingular crest ; cause same as in Cory- 
phodontide. 
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 superior molar. Cause, 
development of anterior basal cingulum of superior molar, which wedges 
Cross-crests of inferior molar anteriorly. 
Fic. 6.—Uintatherium, mandible anterior to coronoid process, one-fourth nat. size; 
from Bridger beds of Wyoming. From Osborn, memoir on Loxolophodon and Uinta- 
therium. 
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 Uintatheriide while it remained 
rudimental in the Coryphodontide? 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 Coryphodontide, it was probably 
ental in Uintatheriide. This explanation is largely hypothetical, yet 
it 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 con- 
‘tact, so that none of them have use on the internal half of the crown 
except the anterior cingular. In the ental movement, on the other hand, 
the limbs of the external V’s are used to the utmost. The posterior limb 
of the anterior V is most used in Pantolambda, for the reason, as it appears 
to me, that the inferior molar is wedged forwards as it moves outwards in 
consequence of the guidance of the anterior cingular crest, and the wedge- 
shape of the triangular superior molar. While this causes the greatest 
use of the posterior limb of the anterior external V, it withdraws the pos- 
terior crest of the inferior molar from shear with the anterior crest of the 
posterior V, so that it has disappeared through disuse. 
In general it may be observed, that the ental movement is the easier to 
the Dinocerata because the V’s open exteriorly in both jaws. In the Pan- 
todonta the ectal movement is easier, because the V’s of the lower molars 
open interionly. 
