240 COPE. [Vot. III. 
nally moderate proportions in Pantolambda (Fig. 62), and from 
the unknown ancestor of that genus, where its dimensions are 
presumably still less considerable. 
Third, the anterior external tubercle or V is reduced to a coni- 
cal rudiment (Fig. 63 a). This is evidently due to the disuse fol- 
lowing the great development of the anterior cingulum which 
extends from the internal tubercle to the anterior external angle 
of the crown. A similar but less considerable development of 
this ridge is accompanied by a corresponding reduction of the 
anterior external lobe, in some genera of the Lophiodontid 
Perissodactyla. The reason why this V has been extinguished 
and not merely pressed backwards, is the fact that the posterior 
external V of the superior molar has retained its place, and has 
not given away to allow room for the anterior one. This V has 
retained its place partly on account of its remoteness from the 
source of pressure in front, but principally because it fits the pos- 
terior transverse crest of the lower molar in front, and the an- 
terior oblique crest of the next succeeding lower molar behind, 
so that its use has been only possible in its primitive position. 
Fourth, the posterior limb of the posterior external V of the 
superior molar is wanting on the last molar in Coryphodon, and 
from the last two in Metalophodon (Fig. 63). The absence of 
this crest from the last superior molar is due to the absence of a 
corresponding crest of the inferior molar (Fig. 64). This is the 
oblique crest at the anterior extremity of the inferior molar, and 
it shears against the posterior limb of the posterior external V 
of the superior molar, representing the sectorial blade of Car- 
nivora. It is little elevated in the Coryphodontidz, owing to 
the fact that it is little used, since the crests of the inferior 
molars shear backwards and not forwards on those of the upper. 
The effect of this disuse tends, in the history of the Corypho- 
dontidz, to become more and more evident. The non-existence 
of a fourth molar behind the third in the lower jaw, accounts 
for the absence of the crest in question from the last superior 
molar, while the absence of the same crest from the second 
superior molar of Metalophodon, indicates the absence or rudi- 
mentary condition of the corresponding crest of the correspond- 
ing inferior molar (Fig. 65). 
The above four propositions cover the principal peculiarities 
of the dentition of the Coryphodontide. I now proceed to a 
consideration of those of the Uintatheriidz. 
