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482 DENTAL ANATOMY.
which the cementuiii has been removed. AUhough the respective pat-
terns are very much alike in their general structure, the differences con-
sist in this : the external cusps of the superior molars are relatively-
larger, more perfectly crescentic, and strongly inclined inward in Hip-
potherium. The anterior cross-crest is better developed and joins the
posterior cross-crest, so as to enclose a deep pit or valley between it and
the antero-external cusp, which is tilled with cement in the natural
Fig. 261.

Fig. 262.
ai ai' P}
jt

OB
i
A Superior Molar Tuutli of a species of ////)- Lower Molar of same. Letters as in Fig. 259.
pollmrmm, withcementum removed : a, an-
terior; 6, posterior; c, internal; borders. Vertical view, natural size.
state ; this is called the anterior lake in the worn tooth. The posterior
cross-crest bends around to join the posterior cingular cusp, which, with
the postero-external cusp, furnishes the boundary of the posterior lake.
To these w'oss-crests are added a greater or lesser number of vertical
folds, which give the borders of the lakes a crenate appearance when
the crown is much worn. The internal cusps are relatively small, the
posterior being connected with the corresponding cross-crest, the ante-
rior isolated. To all these must be added the increased height and the
presence of cementuni.
The lower molars (Fig. 2(32) do not exhibit such marked diiference
from the Anchitherium type as do those above, but they are neverthe-
less more complex in their increased dej^th, complete isolation of the
accessory antero-internal cusp, and the addition of cementum. The grind-
ing surface of the teeth resulting from this arrangement of the enamel,
dentine, and cement is kept constantly rough by reason of the inequalities
in the rate of wear which these substances sustain during mastication.
Coincident with this structure of the crown the roots disappear and the
tooth grows continuously—a condition necessary to compensate for the
great waste of the tooth-substance.
Ijastly, we come to the modern horse, in which digital reduction has
reached the extreme ]ioint in this series, or that furthest removed from
the pcntcdactyle Condi//(irfJira. As is well known, the digital formula
in this family is 1 — 1 in functional use, with the second and fourth
represented by the rudimentary metapodials commonly known as the
" splint bones."
The incisors are peculiar and characteristic, inasmuch as the working
face is interrupted by a deep ])it caused by the upward growth of the
posterior cingulum. Previous to extrusion, the posterior wall of this
cavity is incom])lete and does not rise so high as the anterior.^ After
1 Ryder, "On the Origin and Homologies of the Incisors of the Horse," Proc. Acad.
Nat. Sci., Philada., 1877.