ents upon which the genera and species of the
palaeontologist are so often based. M. Gaudry has arranged the species
of _Hyaenidae, Proboscidea, Rhinocerotidae_, and _Equidae_ in their
order of filiation from their earliest appearance in the Miocene epoch
to the present time, and Professor Ruetimeyer has drawn up similar
schemes for the Oxen and other _Ungulata_--with what, I am disposed
to think, is a fair and probable approximation to the order of nature.
But, as no one is better aware than these two learned, acute, and
philosophical biologists, all such arrangements must be regarded as
provisional, except in those cases in which, by a fortunate accident,
large series of remains are obtainable from a thick and wide-spread
series of deposits. It is easy to accumulate probabilities--hard
to make out some particular case in such a way that it will stand
rigorous criticism.

After much search, however, I think that such a case is to be made out
in favour of the pedigree of the Horses.

The genus _Equus_ is represented as far back as the latter part of the
Miocene epoch; but in deposits belonging to the middle of that
epoch its place is taken by two other genera, _Hipparion_ and
_Anchitherium_[1]; and, in the lowest Miocene and upper Eocene, only
the last genus occurs. A species of _Anchitherium_ was referred by
Cuvier to the _Palaeotheria_ under the name of _P. aurelianense_. The
grinding-teeth are in fact very similar in shape and in pattern, and
in the absence of any thick layer of cement, to those of some species
of _Palaeotherium_, especially Cuvier's _Palaeotherium minus_, which
has been formed into a separate genus, _Plagiolophus_, by Pomel. But
in the fact that there are only six full-sized grinders in the lower
jaw, the first premolar being very small; that the anterior grinders
are as large as, or rather larger than, the posterior ones; that the
second premolar has an anterior prolongation; and that the posterior
molar of the lower jaw has, as Cuvier pointed out, a posterior lobe of
much smaller size and different form, the dentition of _Anchitherium_
departs from the type of the _Palaeotherium_, and approaches that of
the Horse.

[Footnote 1: Hermann von Meyer gave the name of _Anchitherium_ to _A.
Ezguerrae_; and in his paper on the subject he takes great pains
to distinguish the latter as the type of a new genus, from Cuvier's
_Palaeotherium d'Orleans._ But it is precisely the _Palaeotherium
d'Orleans_ whi