ORAL CAVITY 



229 



its conical shape and single root. Behind the canines come the 

 premolars (the bicuspids of the dentists) which have two roots and 

 complicated crowns and appear in both milk and permanent denti- 

 tions. Lastly are the molars, like the premolars in form, with several 

 roots, but appearing only in the permanent dentition. The corre- 

 sponding teeth in the lower jaw have the same names. 



In a few mammals, like the whales, all of the teeth are of a simple conical 

 shape, but in the majority the crown of the molars is marked by projections — 

 cones, tubercles, crests, etc. — which are variously arranged. When the teeth 

 are adapted for cutting they are called secodont (cats, fig. 239); for crushing, 

 btmodont (man); when marked by transverse ridges, lophodont (elephants); 

 when there are longitudinal crests, more or less crescentic in outline, they are 

 selenodont (horse, fig. 241, E). 



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 



Fig. 241. — A , triconodont tooth of Dromatherium; B, tritubercular tooth of Spdaco- 

 Iherium; 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); pa, paracone (paraconid); pr, protocone (protoconid) ; /, talon. 



a metacone behind. In the lower jaw the corresponding terms are proto-, para-, 

 and metaconid. In 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 develops into a promi- 

 nent 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 re- 

 ductions 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, premolars and molars of man are represented by 



