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TEETH OF THE VERTEBRATA. 425

are like those of Stypolojjhus in having the fonrth premolar and all the
true molars tritubercular. There is no broad ledge external to the outer
cusps, however, as in that genus, and the posterior external angle of the
crown is not produced. It is from the White River Miocene of this
country, as is also a nearly-related genus, Ictops of Cope. The only
difference between these two genera is the more complex form of the
fourth superior premolar in the latter.
The living genus Centetes, or tenrec of ]Madagascar, is closely related
to this family, and differs only in the incomplete condition of the zygo-
matic arch. The number and form of the teeth are very like those of
Leptktis, and it is highly probable, as Cope suggests, that Centetes is
the living descendant of this genus.
Another quite remarkable genus which Cope places in this family is
from the Eocene, and was
described by him under the Fig. 207.
name of Esthoni/x. Its
dental formula (Fig. 207)
is 1.1 C.i-,Pm.|, M.f
= 34. The single superior
incisor of each side is great-
ly enlarged, and exceeds the
canine in size. The first pre-
molar is small and has a
simple crown. The next is
larger, and is tritubercular.
The third is like the true
molars, with the exception
that it lacks the internal
cingulum. The true molai's
have two external cusps,
bordered npon the outside
by a broad ledge which is
produced anteriorly into a
marginal cusp. There is
a large internal cusp, from
Esihonyx, burmeisteri, Cope: a, b, c, parts of upper and lower
which is developed at its
jaws, two-thirds natural size (after Cope).
inner posterior extremity
a strong cingulum, the representative of the fourth cusp, which is con-
tinued thence around the base of the crown behind to join its broad
external portion. This is another case wherein the tritubercular and
quadritubercular question is involved in uncertainty.
In the lower jaw the median incisors are small, the outer pair
enlarged, almost equalling the canines. The first two premolars are
small, the third larger, resembling the true molars somewhat in form.
The molars support two Vs, of which the anterior is most elevated.
An analysis of the crown shows it to be of a modified tuberculo-secto-
rial nature, wherein the three anterior cusps are connected by ridges
that extend quite to their summit and form the anterior V. The broad
heel displays two cusps connected by a strong ridge ; from the outer of
these, again, another ridge passes obliquely forward to join the internal