J90 
PAXTODONTA. 
anterior external angle. The tooth, therefore, presents two nearly parallel 
transverse crests, separated by a deep valley, wliich is represented on the 
two preceding molars by the anterior ledge. The posterior limb of the 
posterior V has disappeared, the anterior edge of the two V’s being only 
represented in the posterior crest. In the species with which I am acquainted, 
there is, in addition, a strong anterior basal cingulum on the true molars. 
The inferior true molars present us with two cross-crests, which are 
oblique to the direct axes of the crown, because they connect cusps which 
alternate in position. No oblique crest connects the alternating cusps across 
the valley thus created, although a rudiment of it appears; but a correspond¬ 
ing crest at the anterior extremity of the crown forms a V with the anterior 
cross-crest It, however, descends rapidly to the inner side, where it termi¬ 
nates at the base of the crown. There is no fifth lobe of the last molar, 
but frequently a cingulum-like heel. The premolars differ from the true 
molars in the absence of the posterior cross-crest, and the development of 
the anterior descending crest so as to be equal to the anterior cross-crest, 
thus forming a V with its back inward and apex outward, like the V of the 
superior premolars, but reversed in position. The base of the crown is con¬ 
tinued a short distance behind the V, and bears a low longitudinal median 
crest, which represents the rudimental ridge which connects the cross-crests 
in the true molars. 
The milk dentition is almost completely preserved in a right maxillary 
bone, which I discovered among a large number of other bones, including 
teeth of adult and young Coryphodons, and which I described in the Appen¬ 
dix FF, Annual Report of the Chief of Engineers, 1874, p. 597. The crown 
of the first premolar is injured, but its base has the form of that of the second, 
and it is probable that it had the construction of the latter. The crown of 
the second premolar is, like that of the adult, composed of one external and 
one internal V, but wider in the antero-posterior direction, so that the limbs 
spread more than in the adult. There is an external vertical ridge at the 
posterior part of the posterior limb of the V, which gives a small angle in 
the grinding-surface on attrition.* The third molar is totally unlike the sec¬ 
ond, or the corresponding tooth of the permanent dentition. While it pre¬ 
sents a single internal angular cusp, with angles extending anteriorly and 
