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TEETH OF THE VERTEBRATA. 477

whereas in the Condylarthra the astragakis articulates distally with the
navicular or scaphoid only—a condition which obtains in nearly all
Mammalia. Two prominent divisions of this order can be recognized
—the Artiodactyln, or " split hoofs," of which the hog, cow, and deer,
etc. are familiar examples, and the Perissod((cti/l((, whose only living
representatives are the horse, tapir, and rhinoceros.
The latter sub-order is divisible into a number of sections, which,
when we consider the extinct forms constituting at least nine-tenths of
the species, we are not able to separate by any characters of very great
anatomical importance, notwithstanding the fact that the extremes of
the several stems are different enougli. That family which stands
nearest to the Condylurtlira is the Lopluodontidcv, a group ol" extinct
generalized perissodactyls from the Middle and Lower Eocene beds.
The digital formula is not so great as in the Condylarthra, being only
4— 3, and in one instance 3—3 ; that is to say, four toes on the anterior
and three on the posterior limbs.'
Hyracotherium (Fig. 256) is a typical example of this family, or at
Fig. 25(5.

Skull of Hyracotherium augustidens, Cope, from the \Vind River Beds of Wyoming (after Cope).
least that section of it whose dentition approaches nearest to Phenacodus ;
and if it were not known that the carpal and tarsal articulations were
different, they might easily be mistaken for the same family, so great is
the resemblance of their teeth. The dental formula of this animal is
I. 3. C. J-, Pm. ^, M. f = 44, the same as Fhniacodns. The ]>remolai-s
are'^'different from the molars, being simpler in form, and the first in
both jaws is separated from the others by a diastema. Tlie molar pat-
iProf. Marsh has described several genera of this group, wliich he has called Eohip-
pus, Orohippux, etc., but the descriiitions are so brief that it is impossible to form any
correct estimate of their true relationship. Enhippii.^. he says, has five toes, but further
than this its osteology has not been described. It would be interesting to know in what
respects it diflers from the phenacodonts, Hyracotherium, Pliolophus, etc.