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 



FIG. 216. A, triconodont tooth of Dromatherium; B, tri tubercular tooth ofSpalaco- 

 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) ; t, 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 



.2127 .1:174 



i-, c-, pm-, m-; that of the opossum by i 2 , c-, pm-, m-. 



2123 4134 



