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TEETH OF THE VERTEBRATA. 401

slightly increased size and the possession of a well-defined second
posterior basal lobe. It slightly overlaps the great first true molar
behind it.
The true molars of the superior series are two in number upon each
side, and are placed directly behind the premolars. The definition of a
true molar tooth is one which, being situated behind the premolars, does
not displace a deciduous or milk predecessor. The two molars above
are three-rooted, with broad tuberculated crowns imperfectly quadran-
gular in outline. The first, which is more than twice the size of the
second, has two strong obtuse conical tubercles on the external portion
of the crown, situated directly over the anterior and posterior external
roots ; they are subequal and separated from each other by a transverse
notch. Internal to these there is a broad ledge, well rounded oif inter-
nally, bearing three cusps. The one most internal is lunate in form, and
is closely connected with the cingulum, which surrounds the base of the
tooth. The cusp situated near the antero-internal angle is the largest,
and has a subtrihedral form. A distinct ridge passes outward and for-
ward from it to join the cingulum. Posterior to this last-mentioned
cusp, and separated from it by a wide open notch, is the third tubercle,
less distinctly marked than either of the others. An analysis of the
various cusps of which the crown is composed leaves little room to
doubt that the two external cusps are strictly homologous with the two
external ones of the sectorial in advance of it—that the internal ledge
which bears the three tubercles represents the greatly enlarged internal
lobe of the sectorial, Avhich has been removed to a more posterior
position, and has acquired an important addition from the cingulum.
That part of it which is exactly homologous Avith the internal lobe
is the principal cusp at the antero-internal angle, which in some car-
nivorous animals is continued outward and backward as a prominent
ridge, and does not develop the third tubercle. If the lunate cingular
cusp be subtracted, the crown will be seen to resemble that of the
sectorial in its general features.
The second true molar is like the first, except that the internal ledge
exhibits, instead of three tubercles, two crescentiform ridges.
The first true molar of the lower jaw is the largest tooth in the entire
dentition of the dog, and is the sectorial of the inferior series. It is
implanted by two powerful roots at a point about midway between the
anterior extremity and the condyle of the lower jaw, and occupies a
position near the canthus or angle of the mouth. Its crown may be
described as composed of two anterior blade-like cusps, a small inter-
nal tubercle, and a low tubercular heel. Of the two anterior cusps, the
posterior is the larger, and rises gradually above the level of the one
anterior to it ; both are convex internally, but somewhat flattened exter-
nally to correspond with the internal flattened surface of the two blades
of the superior sectorial. They are separated from each other by a deep,
narrow fissure.' The heel is low, and occupied by two cusps disposed
transversely, of which the outer one is the larger, A faint ridge con-
nects them, enclosing a shallow basin in front ; on this account the heel
is said to be basin-shaped. The internal tubercle is small, and is placed
at the inner posterior part of the median lobe. In many carnivores it
Vol. I.—26