260 COPE. [Vot. Ii. 
still opposite. This peculiarity of the inferior molars is due 
to the development of intermediate tubercles in the superior 
molars, in front of the line which could connect the external 
and internal cusps, as has been already described. These 
appear in a rudimentary condition in Hyracotherium, and pre- 
sent an increasing development in all the lines of the suborder. 
The mechanical effect produced by them on the opposing 
(inferior) molar teeth is as follows. An external cusp of the 
inferior molar in passing across the face of the superior molar 
(ental) is deflected forwards so soon as it strikes the intermediate 
Figure 82.— Protapirus priscus Filhol, superior and inferior molar teeth, grinding 
faces; from the Eocene phosphorites of Quercy; from Filhol. 
tubercle. In this position its apex is nearly opposite to the cor- 
responding external and internal cusps of the superior molar. 
As the inferior molar is narrower than the superior, its interior 
cusps become engaged with those of the superior molar, as the 
external cusps of the former reach the intermediate cusps of 
the latter. Now were the internal cusps of inferior molar 
directly opposite to the external cusps of the same, the former 
would in these circumstances come into collision with the in- 
ternal cusps of the upper series. In order to pass through 
the intervals of the internal cusps of the superior molars, the 
internal cusps of the inferior molars must alternate with the 
exterior cusps of the same. They have been forced into this 
position by the constant pressure on and by the internal cusps 
of the superior molar, which they engage on the posterior side, 
and not on the anterior side, as is the case in types where the 
intermediate tubercles are either in transverse line with the 
