of dentine analogous to that in the leptorhine Rhinoceros (PI. 138,
fig. 7). The posterior crescentic enamel island (c) is a further
development of the fold (c) in the Rhinoceros’ molar, but is much
earlier insulated in the Horse.
In the lower jaw the same analogies may he traced : the teeth
here, as is usual in other quadrupeds, are narrower transversely
than in the upper jaw; they are divided externally into two convex
lobes (PI. 136, fig. 2, m 1, o o) by a median longitudinal fissure,
and on the inner side they present three principal unequal convex
ridges, and an anterior and posterior narrower ridge; but the
crown of the molar is penetrated from the inner side by deeper and
more complex folds than in the Anoplothere and still more so than
in the Rhinoceros or Palseothere. The anterior valley between the
narrow ridge and first principal internal column expands into a
sub-crescentic fold : the second is a short simple fold and terminates
opposite that which penetrates the tooth from the outer side: 'the
third inner fold expands in the posterior lobe of the tooth like the
first; two short folds partially detach a small accessory lobe at the
posterior part of the crown. All the valleys, fissures or folds in both
upper and lower molar teeth, are lined by enamel, which also coats
the whole exterior surface of the crown.
The character by which the Horse’s molars may best be distinguished
from the teeth of other Herbivora corresponding with
them in size, is the great length of the tooth before it divides
into fangs. This division, indeed, does not begin to take place
until much of the crown has been worn away; and thus, except
in old Horses, a considerable proportion of the whole of the molar
is implanted in the socket by an undivided base. This is slightly
curved in the upper molars, the outer side of the bases of which is
shown in PI. 136, fig. 5, the inner side in fig. 6. The deciduous
molars have shorter bodies and sooner begin to develope roots, as
in fig. 5, d 2, 3 & 4 ; but in these, or in an old permanent molar
with roots, the pattern of the grinding surface, as it is shewn in
figs. 1 & 2, though it be a little changed by partial obliteration of
the enamel folds, yet generally retains as much of its character as
to serve, with the form of the tooth, to distinguish such tooth
from the rooted molar of a Ruminant.
Cuvier(l) was unable from the materials at his command to
detect any characters in the bones or teeth of the different existing
species of Equus or in the fossil remains of the same genus, by
which he could distinguish them, save by their difference of size.
Amongst the numerous teeth of a species of Equus, as large as a
horse fourteen hands and a half high, collected from the Oreston
cavernous fissures, I have found specimens clearly indicating two
distinct species, so far as specific differences may be founded on
well-marked modifications of the teeth.
One of these, like the ordinary Equus fossilis of the drift and
pleistocene formations, differs from the existing Equus caballus by
the minor transverse diameter of the molar teeth ; the other, in the
more complex and elegant plication of the enamel, and in the
bilohed posterior termination of the grinding surface of the last upper
molar more closely approximates to the extinct Horse of the miocene
period, which H. v. Meyer has characterised under the name of
the Equus caballus primigenius. The Oreston fossil teeth differ,
however, from this in the form of the fifth or internal prism of
dentine in the upper molars, and in its continuation with the anterior
lobe of the tooth ; the fifth prism p being oval and insulated in the
Equus primigenius of v. Meyer, (Hippotherium, Kaup. PI. 136, fig. 3).
The Oreston fossil molar teeth, which in their principal
characters manifest so close a relationship with the miocene Equus
primigenius, differ, like the later drift species (Eq. fossilis), from the
recent Horse in a greater proportional antero-posterior diameter of
the crown and also in a less produced anterior angle of the first
molar. I have named this ancient British fossil Horse Equus
plicidens(2). The fossil Horse (Equus curvidens) of South America
which coexisted with the Megatherium and, like it, became extinct
apparently before the introduction of the Human Race, differs from
the existing Horse by the greater degree of curvature of the upper
molars. (3).
(1) ‘Ossemens Fossiles,’ 4to. 1822, tom. n. pt. i, p. 111.
(2) ‘ H istory of B rit. Fossil Mammalia, 8vo. p. 392.'
(3) See ‘ Catalogue of Fossil Mammalia in the Museum of the Royal College of Surgeons,’
4to. p. 235. *