214 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
arranged. When the teeth are adapted for cutting they are called 
secodont (cats, fig. 214); for crushing, bunodont (man); when mark- 
ed by transverse ridges, lophodont (elephants) ; when there are longitu- 
dinal crests, more or less crescentic in outline, they are selenodont 
(horse, fig. 216). 
In the triconodont tooth there are three prominences in the crown 
arranged in a straight line, parallel to the axis of the jaw. The middle 
and more prominent of these in the upper jaw is the protocone, with 
a smaller paracone in front and a metacone behind. In the lower 
jaw the corresponding terms are proto-, para-, and metaconid. In 
Fic. 216.—A, triconodont tooth of Dromatherium; B, tritubercular tooth of Spalaco- 
therium; C, interlocking of upper (dark) and lower (light) tritubercular molar teeth (after 
Osborn); D, molar of Erinaceus; E, of horse (selenodont type); c, cingulum; m, metacone 
(metaconid); p, paracone (paraconid); pr, protocone (protoconid); ¢, talon. 
a tritubercular tooth the three cones are arranged in a triangle, in 
such a way that they alternate in the two jaws, the protocone being on 
the inner side, the protoconid on the outer. ‘Tritubercular teeth may 
have a lower projection (talon) on the hinder side. When this devel- 
ops into a prominent tubercle (hypocone, hypoconid) the tooth 
becomes quadritubercular. Then crests or lophs may develop, 
connecting the cones, so that the crown becomes ridged rather than 
tubercular. 
In the homodont dentition the number of teeth may be very large, varying 
from 100 to 200. With the heterodont dentition the number is smaller, the full 
dentition in the placental mammals including 44 teeth. From this number reduc- 
tions may occur by the loss of teeth of any kind. The number of teeth and of 
those of each kind is important in systematic work, and a dental formula has been 
devised to express this. As the number of teeth in the two sides of each jaw is the 
same, only one side is represented in the formula, while the teeth of the upper and 
lower jaws are represented as fractions. The number of incisors, canines, pre- 
molars and molars of man are represented by 
oo FT 2.3 : I 
1 c=,pm-. ue that of the opossum by is, c-,pm§, m4. 
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