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TEETH OF THE VERTEBBATA. 431

primitive cone to fuse into a cutting blade. A single tubercular molar
follows the sectorial, which exhibits, as does the second lower true molar
in the dog, a reduced or degraded condition of the tuberculo-sectorial
pattern.
Miacis has three true molars in the lower jaw, of which the first
is sectorial, in this respect resembling very closely the lower jaw of the
dog ; its complete dental formula is not known.
Tp:eth of the Prosimi.e.—With this group we enter that division
of the Ijuiiotheria which leads out to the monkeys and man. Its palteon-
tological history reveals an antiquity quite equal to that of any other
of the Monodelphia, continuing backward to the lowest Eocene. It
is customary with most naturalists to regard the Proshnke as widely
separated from the Insedh-ora on account of the higher order of brain-
structure which the living representatives of the lemurs display, and
they are accordingly placed near the Primates. Owing to the perish-
able condition and non-preservation of the soft parts in the extinct
forms generally, we will never be able to know the exact structure
of their brains, but must be content to judge of its generalized or
specialized character by the mould of the cranial cavity, which in
many respects is unsatisfactory.
It can be shown in the ungulate series that the lowest Eocene repre-
sentatives possessed brains, judging from the cranial casts, almost as low
in the scale of organization as that of the lowest known mammals, and
it is likewise true that the brains of the Eocene prosiraians were more
generalized than those now living. I do not think there can be any
radical differences shown to exist between the structure of the brain
of such forms as the squirrel shrews [Taupaiadce), the elephant shrews
{]\IacrosGelidcB), and the (Taleopithecidce of the insectivores, and the
true lemurs (^Lemnridce), the fossil Adapts, and others of the Pro-
simice. The very fact of their remote antiquity and appearance in an
age when the brain-development of all the Mammalia was small would
of itself lead to the supposition that they too at first possessed brains of
lowly organization. It should be here stated that very few skulls of
the Eocene prosimians are known.
The dental formula of the spectrum lemur (Tarsius spectrum) is
I. y, C ^, Pm. 1^, M. Of the two pairs of incisors in the upper jaw,
f.
the median is much the larger ; they are closely approximated, long,
pointed, and conical, and are surrounded at the base by a prominent
cingulum, which is well defined upon the anterior face of the crown.
The next pair are much smaller, and also have pointed crowns and
basal cingul-a. The upper canines are about equal in size to the
median incisors, which they resemble both in the form of their
crowns and the cingulum at the base.
The first premolar is the smallest of the three, and is placed just
behind the canine its crown is simple and pointed. The next two are
;
larger and imperfectly two-lobed, the internal lobe being represented by
a strongly-developed cinguhnii which continues around upon the outer
face of the tooth. The true molars are subequal in size and tritubercu-
lar. The two external cusps are well developed, and jilaced at the
external border of the crown. The internal lobe is relatively large.