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TEETH OF THE VERTEBRA TA. 417

series—the prosimian or lemuroicl and the insectivorous ; but if we go
further, and establish another sub-order upon the tritubercular or quad-
ritubereular character of the superior mokir teeth, it will necessitate the
wide separation of forms closely related to each other by every import-
ant feature of their anatomical structure—a course which I do not deem
advisable nor in keeping with our present knowledge of the subject.
According to Prof Cope's definition, the only character in which the
Crei)donta differ from the Insectivora is the tritubercular superior molars
as distinguished from the quadritubercular ; and in order to make the
Creodonta homogeneous he is compelled to take out of the old group
Insectivora the Taupa'iadce, Centetidce, Chrysiochloridce, and Talpidce, and
place them in the Creodonta. Aside from the inadvisability of such a
course, these teeth in many of the above-named families are altogether
intermediate between the tritubercular and quadritubercular pattern, the
postero-internal tubercle being represented often by a rudimentary cingu-
lum, which may be entirely absent or produced into a strong cusp.
Then, again, the superior molar teeth of the prosimian division are
indifferently tritubercular or quadritubercular ; and if we adhere to
this practice in the one, why not in the other? In consequence of
these facts, I propose to unite the Creodonta with the Insectivora into
a single division, fjr which the old name Insectivora may be retained.
Thus constituted, palteontological history, in my judgment, points
strongly to the fact that this group stands in the important relationship
of ancestors to a large part, if not the whole, of the unguiculate Mam-
malia. Working upon this hypothesis, it will be desirable to describe
the more important types of dental structure to be met with in this sub-
order, after which they can be followed out to their respective termina-
tions in the various orders and sub-orders which make up the series.
Insectivora.—The simplest form of dental structure in this sub-order
is exhibited by the extinct genera Mesonyx and Dissacus of Cope, from
the American Eocene strata. The teeth of Mesonyx (Figs. 200, 201)
are forty-four in number, disposed as follows : I. f , C. \, Pm. |-, M. f
= 42. The incisors are relatively small, with subconic crowns, which
are closely approximated. The superior canines are large, recurved, and
pointed, being placed at a considerable distance from the incisors to
accommodate the crown of the inferior canine. The three anterior pre-
molars of the upper jaw are two-rooted, with the exception of the first,
which is probably single-rooted. They have comparatively simple com-
pressed crowns, with a principal cusp and a posterior basal lobe, sur-
rounded by a basal cingulum. The fourth is more complex, and resem-
bles the true molars posterior to it. Like them, it has three principal
cusps, of which two are external and one internal, giving to the crown
a triangular shape. In the first true molar the postero-external angle
of the crown is produced into a strong blade-like process, a develop-
ment of the cingulum which is conspicuous in all. The last molar of
this series is bicuspid, the posterior of the two external cusps being
absent.
In the lower jaw both the premolars and molars are remarkable for
their simplicity. The first premolar is single-rooted, and has a sub-
conic crown, as in the dog. The teeth behind it are two-rooted, and
Vol. I.—27