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TEETH OF THE VERTEBRATA. 435

ally the case in all the Primates, and are in contact in the median
line. The smaller outer incisors follow closely in the dentigerous
border of the premaxillaries, after which there is a wide space, almost
equal to the width of the two incisors, for the passage of the lower
canine. The canines of the upper jaw are comparatively strong for the
monkeys, and have pointed, slightly recurved croAvns which project far
above the level of the other teeth ; there is a deep groove upon their
anterior faces.
The premolars or bicuspids are three in number, and completely fill
the interval between the canines and molars. The first is the smallest,
and has a prominent pointed external cusp on the grinding surface, to
which tlie cingulum adds a low U-shaped internal portion ; the second
and third are similar, except that the internal lobe is no longer cingular,
the cingulum furnishing a second internal ledge. The true molars are
two in number upon each side, in this respect differing from all known
Primates. The only approach to this condition to be met with else-
where in the order is in the dentition of man, in whom it appears, as
we will hereafter see, that the last molar, or the " wisdom tooth," is
gradually becoming rudimentary or defective in the higher races. A^ari-
ous causes have been assigned in explanation of this fact, one of which
is that the greater development of the brain necessitates the expenditure
of smaller amount of growth-force upon the maxillary bones, whereby
insufficient room is allowed and the tooth stunted. If this be the real
cause, it is difficult to understand why in the lowest representatives of
the order—and those, too, in which the cerebral hemispheres are pro-
portionally the smallest—the complete suppression of the last molar
should have occurred.
The two pairs of lower incisors are small and of the usual incisiform
pattern, being consideralily smaller than the canines. The lower incisors
of the allied genus, Hapale, are proclivous, the canines being relatively
small and approximated to them, as in the lemurs, although not to so
great an extent. The canines are almost equal to the upper ones in size,
and follow the outer incisors without interruption. The three lower
premolars are subequal, the summit of the first being elevated above
the level of the succeeding teeth. In the first the anterior basal lobe, the
principal cone, and an imperfect heel can be indistinctly made out, while
in the second and third the internal tubercle is present. In the true
molars there are four indistinct cusps; the anterior basal lobe has almost
completely disappeared, and all the cusps are of equal height. A care-
ful study of unworn teeth will show them to be a still further modifica-
tion of the tuberculo-sectorial type, whereby the perfect quadrituber-
cular has been produced.
The next division, Platyrrhines, or flat-nosed monkeys, constitute the
family Cebidce, in which the dental formula is I. |, C. \, Pm. |, M.
|
= 36. They belong to the continent of South America, and have pre-
hensile tails and generally rudimentary thumbs. The canines are usually
strong and prominent, and the superior molars have a well-defined ridge
connecting the antero-internal with the postero-external cusps, a rem-
nant of the tritubercular condition. This ridge is found with varying
constancy in the superior molars of all the Primates, and marks the