As all the above-mentioned forms
succeed one another in point of time, it may be regarded as probable
that we shall yet be able to point, with some certainty, to some
still older example of the _Equidoe_, in which the first digit
is developed, and the foot assumes its typical five-fingered
condition.
[Illustration: Fig. 230.--Skeleton of the foot in various forms
belonging to the family of the _Equidoe_. A, Foot of _Orohippus_,
Eocene; B, Foot of _Anchitherium, Upper Eocene and Lower Miocene;
C, Foot of _Hipparion_, Upper Miocene and Pliocene: D, Foot of
Horse (_Equus_), Pliocene and Recent. The figures indicate the
numbers of the digits in the typical five-fingered hand of Mammals.
(After Marsh.)]
Passing on to the Even-toed or _Artiodactyle Ungulates_, no
representative of the _Hippotamus_ seems yet to have existed, but
there are several forms (_Choeropotamus, Hyopotamus_, &c.) more
or less closely allied to the Pigs (_Suida_); and the singular
group of the _Anoplotheridoe_ may be regarded as forming a kind of
transition between the Swine and the Ruminants. The _Anoplotheria_
(fig. 231) were slender in form, the largest not exceeding a
donkey in size, with long tails, and having the feet terminated
by two hoofed toes each, sometimes with a pair of small accessory
hoofs as well. The teeth exhibit the peculiarity that they are
arranged in a continuous series, without any gap or interval
between the molars and the canines; and the back teeth, like
those of all the Ungulates, are adapted for grinding vegetable
food, their crowns resembling in form those of the true Ruminants.
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