258 COPE. [Vou. III. 
of junction of the oblique transverse crest with the external wall, 
looks zzwards instead of outwards; the true second external 
cusp is behind this point, as is the first external cusp in the 
same crown. In the inward movement of the inferior molar 
crown over the external wall of the superior molar in mastica- 
tion, the shearing and pressure are of course effected by the 
posterior faces of the transverse crests of the inferior molars 
against the anterior faces of the corresponding crests of the 
superior molar. In crossing the external wall of the superior 
molar, the shearing is on that part which is opposite to the 
‘transverse crest, and from before backwards. The crests of the 
superior molars which are immediately posterior to a transverse 
crest receive no, or relatively little, shearing strain. The result 
is that the part of the wall in front of the posterior external 
crest and cusp is carried inwards by the ectal movement of the 
posterior crest of the inferior molar, while the crest behind that 
cusp, being without such strain, remains either in line with the 
anterior wall ridge, or remaining in its original position poste- 
riorly, is carried inwards with the anterior crest and cusp, thus 
forming an open V._ It is also probable that in this line there 
has been in mastication a rotary movement of the inferior mo- 
lars on the superior. The ectal has evidently been a backward 
movement as well; if so, the ental movement would be a for- 
ward one. Such a movement would at once account for the 
outward flexure of the posterior free extremity of the external 
wall of the superior molar (Fig. 80) ; since it would in that case 
receive the outward shear of the anterior part of the inferior 
molar behind and below it. The same movement would account 
for the external extension of the anterior cingulum of the 
superior molars, which is such a characteristic peculiarity of the 
Lophiodontidz and of their Rhinocerontine descendants. 
The inferior molars of the Perissodactyla develop from the 
primitive quadritubercular type in two directions, the one anti- 
odont, the other amcebodont, as already stated. In the former 
the cusps unite to form cross-crests; in the other the junction 
of the cusps forms a W or double crescentic pattern. We have 
the former in some members of the Lophiodontidz, in the 
Tapiride, and in the Rhinocerontine series generally. The 
latter type is characteristic of the Equine series. 
The origin of the crests is, in accordance with I. (p. 218), sim- 
