228 COPE. . L VoL. TLE: 
between those of opposite jaws. As the lesser cusps are the 
less resistant to the wedging pressure of such contact, their 
position would change under its influence, rather than the large 
central cusps. The lower jaw fitting within the upper, the effect 
of the collision between the cusplets would be to emphasize the 
relation still more ; that is, the cusplets of the upper jaw would 
be wedged outwards, while those of the lower jaw would be 
pressed inwards, the major cusps retaining at first their original 
alternate position. With increase of the size of the teeth the 
cusps would soon assume in each jaw a position more or less 
transverse to that of the other jaw, producing, as a result of the 
crowding, a crown with a triangular section in both. The pro- 
cess may be rendered clear by the following diagram : 
ae eee ve Ned 
A B G; 
Figure 53.— Diagrammatic representations of horizontal sections of tricuspidate 
molars of both jaws in mutual relation; the shaded ones represent those of the upper 
jaw: Fig. 4, Triconodon; Fig. 2, Menacodon; Fig. C, ideal tritubercular molars, 
approached by Menacodon, Fig. 52. 
The first modification of the tritubercular molar of the lower 
jaw is the addition of a low cingulum at the posterior base. 
This is seen in a rudimentary condition in various living species 
of the Centetidz and Chrysochloridide of the Insectivorous | 
order (Fig. 56); but in these existing forms the superior molar 
has added a posterior cingulum also, which widens internally, or 
towards the palate (Fig. 54). In the evolution of the dentition, 
the inferior posterior cingulum, or “heel,” was developed first, 
as in the Deltatherium, Centetes, and Stypolophus (Figs. 54, 56, 
58), where it is quite large; while the superior cingulum is want- 
ing in Stypolophus and Didelphodus, but is present in a very 
rudimentary condition in Deltatherium fundaminis. In all of 
these genera the external cusps of the superior series have been 
pressed inwards, and more or less together, and are therefore 
removed in this respect from the primitive condition. The more 
primitive state of the superior cusps is seen in some species of 
Miocleenus, where, however, the posterior cingulum is devel- 
oped. The primitive type of tritubercular superior molar is that 
of Sarcothraustes, and in the same genus the inferior molar only 
