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418 DENTAL ANATOMY.
have a general premolariform appearance, the true molars exhibiting but
little departure from the conical pattern of the lower Vertebrata. As in
the premolars of the dog, their crowns are laterally compressed, of a
Fig. 200.

Mandible of Mesonyxossifragns, Cope, from tlie Wasatch Epoch of the Big Horn River, Wyoming, one-
third uaturiil size (after Cope).
triangular form when viewed from the side, having a principal median
cusp, to wliich are added an anterior and posterior smaller one from the
cingulum.
It is a matter of considerable interest to find in this ancient represen-
tative of the unguiculate series so simple and generalized a dentition,
inasnuich as it furnishes a key to an interpretation of the lobes and
cusps of the teeth of many of the succeeding forms. It is more than
probal)]e tliat tin's ])nrticular species is not the original ancestral form
from which the otlicrs have l)een derived, on account of certain charac-
ters presented by the skeleton, but, as far as the teeth of the lower jaw
are concerned, they exhibit just such a transitional condition between
the primitive cone of the theromorjih R,e]itilia and the lowest forms
of mannnalian teeth as we would most reasonably expect to find in
the ]irimitive ancestor.
The various steps in this jirocess of dental evolution I conceive to
have been as follows: (1) additions to the anterior and posterior edges
of the cone and tlie formation of a cingulum ; (2) division of the single