318 
PERISSODACTYLA. 
front. The latter is slightly concave in the antero-posterior direction, ex¬ 
cepting in the first and second premolars and third true molar. On con¬ 
tinued attrition, the transverse valley of the crowns becomes isolated by 
the confluence of the posterior projection of the anterior cross-crest with 
the posterior cross-crest. The posterior emargination scarcely becomes iso¬ 
lated, and then is only inclosed by an enamel band. This is only possible 
on the third and fourth premolars and the first true molar. The sides of 
the transverse valley have no vertical ribs near the fundus or elsewhere, so 
that it does not become lobate, or divided into lakes, on attrition, as is the 
case with Uhinocerus indicus and several species of Atelodus. They have the 
simpler structure seen in the Aceratherium occidentale. 
The mandibular ramus is compressed and not deep. Its inferior out¬ 
line is gently curved, and without irregularity at either extremity. The 
masseteric region is flat, while the internal pterygoid insertion is a concavity 
of the inner face behind and below the coronoid process. The inferior pre¬ 
molar teeth increase regularly in size to the true molars, which are nearly 
equal. In all, but especially on the last two molars, the anterior limb of 
the anterior crescent is well developed, extending to the inner side. The 
external posterior projection of this crescent is rounded. The symphysis 
is short, commencing below the second premolar. Immediately in front of 
this point, it is contracted, and the siij^erior surface forms a trough between 
the acute superior lateral borders. The symphysis is scarcely expanded at 
the extremity, where it supports two larger external and two smaller inter¬ 
nal incisors. 
The mandible differs from that of A. megalodus in the contracted form 
of the symphysis, and the small size of the outer tusk-like incisors, which 
are scarcely half as large as those of the former; but it is possible that this 
character is sexual. As compared with five mandibles of A. megalodus, the 
last molar originates closer to the base of coronoid process. In the latter, 
there is a considerable interval in front of the ascending process. The form 
of the dentary bone is that of A. megalodus, and not thick and massive, as in 
A. crassus. 
The posterior part of the temporal bone is preserved. It supports a 
rather long and narrow postglenoid process. Just behind the superior l)ase 
