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484 DENTAL ANATOMY.

enlargement of the antero-internal lobe and its connection by a ridge
with its corresponding cross-crest. Some species of Hippotherium show
a gradual advance from the conic isolated condition of this element to
its enlarged and sub-connected form.
Thus it is that palaeontology has enabled us to fully comprehend
the different steps in the production of these complex and specialized
organs from the simple bunodont pattern. To say that such evidence
is witliout its special bearing on tlie great problem of biology, or that
evolution or development has not taken place, is to deny the truth of
the assertions herein made. Many intermediate steps between those
given could be cited, but time and space have compelled me to limit
tlie examples to the most salient.
The remaining perissodactyles exhibit different degrees of modification
of the bunodont type, none having reached the same stage of perfection
as the horse.
The second sub-order of the ungulates, Artiodacti/la, attained its
greatest development at a later geological period, and it is probably in
the present epoch that the genera and species are the most numerous.
A few genera are found in the Lower Eocene, but they are of rare
occurrence as compared with the perissodactyles. It is probable that
they two came off the condylarthrous stem, but the direct evidence to
substantiate this supposition is wanting. They are primarily divisible
into two groups, Bunodontia and Selenodontia, characterized by the
pattern of the molar teeth and the consequent condition of the posterior
termination of the maxillary bones. In the former division, of which
the hog is an excellent example, the molars have approximately the
same pattern as PJicnacodns ; the tooth-line is little curved, and the
posterior extremity of tlie maxillary is apjilied closely to the pala-
tine and pterygoid bones, whereas in the Sc/cnodontia the molar teeth
have crescentic cusps, and the posterior borders of the maxillaries are
separated by a wide sinus from the palatines and pterygoids. These
characters at first appear insignificant and inadequate to establish and
define two such great groups as the foregoing ; but when we remember
that they express a very important structural modification, and that the
two are correlated, we cease to express surprise.
Of these two divisions, the Bunodontia is the older, and as a conse-
quence the more generalized. Their generalized characters are most
conspicuously displayed in the increased number of digits, bunodont
teeth, absence of horns, non-complexity of the stomach, and separate
condition of all the limb bones. In fact, the suilline artiodactyles are as
primitive in many respects as the (hndi/faiihrd, but in the arrangement
of the carpal and tarsal elements they are specialized and far removed
from their ])rimitive ancestrv.
In the hog the dental fornmla is I. |, C. |, Pm. f , M. f = 44. The
outer pair of incisors are small, and sometimes fall out in old age. The
canines are relativ'cly large—dis))n)])()rtionally so in the male—and in
the upper jaw curve round in such a manner that the point of the crown
is directed upward. The enamel of these teeth does not uniformly
invest the crown, but is disposed in three bands corresponding with its
trihedral form. The canines of the lower jaw are more slender and