Page 417 - My FlipBook

TEETH OF THE VERTEBRATA. 427

The two large incisors above are hook-shapecl, with a prominent pos-
terior ledge at the base, which in some species of this tamily is produced
into a strong basal cusp. The next two teeth are much smaller and sub-
equal, while the three following, rapidly decrease in size, the last becom-
ing very small. Exclusi\'e of the large hook-shaped incisor, the above-
mentioned teeth display a principal cone with an internal lolje and
an external cingulum, which is most distinct in the fourth, fifth, and
sixth. The next tooth, which I take to be the last premolar, marks an
abrupt change both in the character and size of the teeth of the ujiper
series. It is almost equal in size to the first true molar, which it resem-
bles very closely in structure.
The crowns of the molars may be described as consisting of four
principal cusps or tubercles, of which two are external and two internal,
and are therefore quadritubercular. The two external cusps stand at the
apex of two Vs, which open externally, giving to this part of the tooth a
distinct W pattern. At the extreme antero-external angle of the crown
there is a considerable cingular cusp, which is connected with the main
antero-external tubercle by a prominent ridge, thereby forming the first
downward stroke of the W. From this main tubercle another well-
marked ridge passes outward and backward to another small cusp of
the cingulum, situated at a point midway between the two main exter-
nal tubercles on the outer edge of the crown, forming the first upward
stroke of the W and completing the first V. The second downward
stroke of the W is furnished by a ridge connecting the small median
marginal cusp ^vith the postero-external tubercle, while the second upward
stroke of the W is formed by a ridge continued outward and backward
to the postero-external angle of the crown, where it terminates in as light
enlargement.
From the apex of each of the Vs a high ridge passes inward, meeting
at the antero-internal angle of the crown, forming a distinct U. The
inner part of this crest is the antero-internal tubercle. The postero-
internal tubercle stands just behind it, and exhibits a somewhat cres-
centiform pattern. It will be seen that the tooth just described does
not differ materially from those of some other insectivores already
noticed, especially Esthonyx. The principal differences are to be found
in the greater development of the marginal cingular cusps and connect-
ing ridges upon the external part of the crown, which we have, in a
measure, foreshadowed in Esthonyx and others.
In the lower jaw the single pair of incisors are large, scalpriform, and
procumbent. The two succeeding premolars are small, single-fanged,
and have simple crowns. The first two true molars are the largest of
the molar and premolar series, and exhibit a structure identical with
that of Esthonyx. The last is very small, and corresponds with the last
tooth of the upper jaw, which frequently disappears. The crowns of
all the teeth are stained a deep wine-color by a pigment which pene-
trates the substance of the enamel, this tissue being remarkable for its
thickness in all the Insectivores.
Considerable discussion has taken place in regard to the nature of the
external Vs and the exact homology of the two external tubercles. Since
this W-structure is common to the superior true molars of all the moles,