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THE MOLABS. 37
lingual tubercles vary much in size, so that one may be suppressed and
the tooth seem bicuspid. The mesial lingual tubercle (e) may be of
large size and be developed at the expense of the distal (d) ; this may
be a mere cingule on the distal marginal ridge and appear on the distal
side, but it is always present.
The morsal groove (e) is triangular in design, passing between each
of the three tubercles. A well-marked triangular ridge descends from
each of the cusps.
The tricuspid form of the morsal surface of this tooth is, of course,
a reproduction of the trituberculate premolars of the lower primates,
and of still lower mammals, although the triangular form of the crown
is lost in man.

The Molars.
14, The Tuberculate Teeth.—Molar teeth appear early in the scale
of vertebrate life ; mere crushing teeth are found in fishes and slightly
tuberculate teeth in the reptiles. The grinders are of simple form in the
lowest vertebrates. The Bruta have simple, flat-crowned molars, which
are undiiferentiated and used merely for crushing. Tuberculate molars
appeared early in the placental mammalia, the trituberculate molars being
found in numerous fossil species, which are the typal form and forerunners
of the tuberculate molars in the liigher mammalia. The simjjle-crowned
tooth with a single tubercle {hap/odonf, Cope), becomes duplicated and
doubled, with a crown supporting several tubercles [hunodont). The
transition from simple to complex teeth is accomplished by the repeti-
tion of the type in diiferent directions and the addition of cusps and
roots both laterally and longitudinally of the jaw. The upper molar is
formed by the addition of the third cusp to the bicuspid type and has
three roots, which support three or four tubercles. Lower molars con-
sist of four cones which support four or five tubercles. The low^er mo-
lar is therefore the more complex tooth. The bicuspid is more complex
than the canine, the upper molar than the bicuspid, the lower than the
upper molar.
The molar teeth of man are bunodont in form, /. e. thev have simple
rounded tubercles on the grinding face. They are of simple and primi-
tive type, and indeed are most like the molars of the bears and other
omnivorous animals. They are not highly specialized like those of the
carnivora on the one hand with high sharp blades for cutting flesh, nor
like those of the herbivora on the other, which are extended laterally for
grinding tough vegetable fibre. They are of low organization as regards
their functional development.
The molars in man are twelve in number, three on each side of each
jaw, and are placed at the rear of the arch, opposite the strong triturat-