244 COPE. [Vot. III. 
except the anterior cingular. In the ental movement, on the 
other hand, the limbs of the external V’s are used to the utmost. 
The posterior limb of the anterior V is most used in Panto- 
lambda, for the reason, as it appears to me, that the inferior 
molar is wedged forwards as it moves outwards in consequence 
of the guidance of the anterior cingular crest, and the wedge- 
shape of the triangular superior molar. While this causes the 
greatest use of the posterior limb of the anterior external V, it 
withdraws the posterior crest of the inferior molar from shear 
with the anterior crest of the posterior V, so that it has disap- 
peared through disuse. 
In general it may be observed, that the ental movement 
is the easier to the Dinocerata because the V’s open exteriorly 
in both jaws. In the Pantodonta the ectal movement is eAsier, 
because the V’s of the lower molars open interiorly. 
9g. THE ORIGIN OF THE QUADRITUBERCULAR MOLAR. 
The quadritubercular molar of the upper jaw was produced by 
the addition of a tubercle, the hypocone, at the posterior side of 
the internal tubercle of the original tritubercular crown. This 
appears first as a cingulum in the Condylarthrous family of 
Figure 68.— Phenacodus primevus Cope; diagram representing relation of supe- 
rior and inferior molars in mastication; lettering as in Fig. 58. 
the Periptychidz and the Creodont genus Mioclzenus, as ex- 
amples. A cusp rises on the inner part of this cingulum, which 
is at first rudimental, as may be seen in M/voclenus corrugatus 
and Chriacus truncatus as examples. It acquires in successive 
forms increased proportions, as may be seen in the Phenaco- 
dontidee (e.g. Phenacodus primevus, Fig. 68). In the inferior 
molar the quadritubercular is formed by the loss of the para- 
conid, and the development of two hypoconids on the oppo- 
site angles of the heel. The paraconid diminishes farz passu 
with the increase in size of the superior hypoconid (fourth tub- 
