MAMMALS. 365 



simple cone or chisel. The molars vary greatly in shape and 

 structure, and may have several roots, a feature not found in 

 other living vertebrates. 



The shapes and modifications of the molars are of great 

 value in classification, and a few definitions may prove of use 

 in reading the descriptions of systematic works. In the more 

 primitive teeth, each tooth, no matter where placed, has the shape 

 of a simple cone, as in the denticete whales (haplodont), but usu- 

 ally the crowns of the molars present crests, prominences, tuber- 

 ctc. There are two views as to the origin of the more 

 complex condition. According to the one, the typical mam- 

 malian molars have arisen by the fusion of several simple teeth, 

 like those of many reptiles. The other view is that accessory 

 prominences have been developed upon the primary tooth, a view 

 which has much in its favor. First to appear 

 ot these more complex teeth was the tricono- 

 dont, in which secondary prominences cones 

 in the upper jaw, conids in the lower were 

 leveloped in a straight line, a paracone (para- FIG. 350 

 conid) in front of the primary or protocone, and Bunodont tooth 





a metacone behind. In the tritubercular tooth, " 



which came next in turn, the three cones are 

 arranged in a triangle, the protocone on the inner side, the pro- 

 toconid on the outer. This part of the tooth forms the trigon, 

 and from this modifications may be developed in different direc- 

 tions. Thus, while retaining its tubercular character, a posterior 

 lower heel or talon may be formed, and when this develops a 

 single tubercle it is known as the hypocone or hypoconid, the 

 former occurring at the postero-internal angle of the upper 

 molars, the latter at the postero-external angle of the lower. 

 Protoconule and metaconule are smaller intermediate cusps, 

 while the crests which connect the cones and conids are known 

 as lophs. Again, peripheral cusps or styles may arise outside 

 these from the girdle or cingulum of the tooth. 



The tubercles and lophs of the teeth also vary in character. 

 When the surface is calculated for cutting, the tooth is secodont, 

 when for crushing, it isbunodont; with the development of prom- 

 inent transverse crests the tooth becomes lophodont. When the 



