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420 DENTAL ANATOMY.

movement, as the greatest pressure was in front of or behind it. This
would cause the stimulation of the anterior and posterior faces of the root,
and as a consequence of this a vertical groove was first formed upon each
side, which eventually coalesced, dividing the root into two. As we
have already seen, this condition is found in a theromor})h reptile,
and is likewise to be found in the premolars of many existing animals.
The development of basal cusps would naturally follow at those points
where the crown sustained the greatest amount of resistance, which
would be at the base of the triangle.
It is a rule of pretty general application in heterodont teeth that the
molars are more modified than the premolars. This, in all probability,
results from the greater mechanical advantage which is gained by bring-
ing the morsel to be crushed or divided to the posterior part of the
mouth ; that is to say, the resistance as near to the power as possible.
The power in this case is the muscles which close the mouth, which,
being attached to the posterior part of the jaw, exert the greatest
influence upon those teeth in the vicinity (3f their attachment.
The next step in dental complication is seen in the genus Dlssacus,
from the lowest Eocene, the lower teeth of which are represented in
Fig. 202. They are very similar in general appearance to those of
Fig. 202.

Bissncus naviijoviiis, Cope. Right Alandibulnr Pianius, three-fourths natural size: a, external ; 6, supe-
rior view, Iroui the Puerco Beds of New Mexico (after Cope).
Mesonyx, with which they also agree in number. There is, however, an
additional cusp developed upon the inner side of the median cone near
its summit, which is the homologue of the internal tubercle of the infe-
rior sectorial of the dog, as well as that of many other animals of the
unguiculate series. The upper teeth are not known. The genera in
which tlie mandibular teeth ]n'esent this premolariform structure are
associated by Cope into a family which he calls the Jlcsonj/cJiiflce.
As a probable derivative of this family we have the extinct family
Ui/cenodontidce, of which the teeth of the single genus Hycenodon are
represented in Fig. 20.3. This animal is known, so far, from the Mio-
cene dc])osits of this country and Europe only, and has been shown by
Prof AV. B. Scott of Princeton College to be a near relative of Mesonyx.
The dental formula is, according to Gaudry, I. |, C. Pm. ^, M.
-f, f
= 42. The incisors resemble those of the dog, the median pair being