Page 115 - My FlipBook

OF THE GRINDERS. 99
conical : it is much smaller than the rest of the Grinders. In
the Upper-Jaw this Tooth has more variety than in the Lower,
and is even smaller than the corresponding Tooth of the Lower,

developed from the inner instead of the onter side of the crown. Com-
pared with the molar of the Anoplotherinm, the outer lobes of that of
Dichodon are thicker and sharper, the inner ones are more nearly equal
to the outer, and are more distinctly divided from them ; the transverse
valley extends completely across the tooth, and is crossed by the
antero-posterior doubly crescentic depression. In existing Ruminants,
the cusps of the upper molars are . less pointed and lower than in
Dichodon. When worn, the crown presents the inner and outer pair
of crescentic lobes of dentine, whilst the double crescentic valley
separating them, containing a thicker layer of cement, forms two
detached crescents. The premolar resembles one half of the true
molar. The permanent formula of the grinding series in existing Rumi-
nants is p. |; |; , m, f \ l\ . The lower molar of the Ruminant resembles
the upper one reversed. The single median longitudinal depression
which in the upper tooth is internal, in the lower is external ; whilst
the two concavities of the outer side of the upper molar are repeated,
although less deep on the inner side of the lower. The lower molars
are thinner than the upper, and in the worn crown, the crescentic
islands are narrower and less bowed. The outer contour of the grinding
series in Ruminants is slightly zigzag, the anterior and outer angle of
each tooth projecting beyond the posterior and outer angle of the tooth
before it. The premolars form a continuous series with the molars. They
are smaller and more simple.
In the Hyracotherium, another eocene herbivore, the grinding sur-
face supports four principal cusps. Each transverse pair is connected
by a ridge which supports a smaller conical tubercle, and the crown is
girt with a cingulum. On this type are formed the upper grinders of
the existing Hog tribe and of the Hippopotamus. The genus Sus is an
instance of the retention of the typical formula p. \\ %\ , m.% \ || . In the
Wild Boar, the teeth of the grinding series increase in size from the
first to the last. The first premolar has a simple, conical crown, thickest
behind, and is implanted by two fangs. The second has a broader
crown, with a posterior lobe, having a depression on its inner surface ;
each fang exhibits a tendency to division. The crown of the third is
still broader, and it is implanted by four fangs. The fourth premolar
has two principal tubercles, and some irregular vertical depressions on
the inner half of the crown. The first true molar originally bears four
principal cones, with smaller irregular sub-divisions, and an anterior
and posterior ridge ; but owing to its early development, the tubercles
become worn down, and a smooth surface of dentine is exposed by
the time the last molar is in place. A crucial depression, dividing four