THE TEETH 445 



The references on the upper teeth are : pr, protocone ; pa, paracone ; me, metacone ; hy, hypocone ; 

 prl, protoconulc ; mel, metaconule ; tr, tritocone; d, deuterocone ; te, tetartocone; ps, protostyle. 

 On the lower teeth, pr d , protoconid ; pa d , paraconid ; me d , metaconid ; hy d , hypoconid ; en d , cnto- 

 conid ; hyl' 1 , hypoconulid ; d a , deuteroconid ; te d , tetartoconid. 



A-C, horizontal section of tricuspidate molars of both jaws in mutual relation. A, Tricono- 

 don ; B, Menacodon; C, ideal tritubercular molars approached by M</n/<-u</u, t , B. (After Cope.) 



D-H, successive modifications from the triconodont, D ; the double tritubercular, E ; the car- 

 nivorous tritubercular, F ; to the less and more complete quadritubercular, G, H. (From O.-boni.) 

 In each stage the upper figure is the lateral view of the teeth and the lower figure is the diagram 

 of the apposition of the surfaces of the crowns. The genera represented are: Tricin><l<>n, D; 

 Peralestes and SpalacoHierhim, E; Didymictis, F; Mioclcemis, G; Hycpsodus, H. 



I, the apposition of the inferior and superior molars of Stypolophus Whitice (Cope), one of the 

 Creodonta, the ancestors of the Carnivora. (After Cope.) Only the true molars are numbered. 



J, the apposition of the cheek teeth of the Cat; premolars and molars numbered. 



K-V, evolution of the premolars from the haplodont to the quadritubercular type, according 

 to Scott ; K-O, upper premolars ; P-V, lower premolars. 



put. Thus severe pressure on a simple tooth crown would, if long 

 continued, cause it to expand laterally, or in the direction of least 

 resistance, and to grow but little in its vertical axis, i.e., in the 

 direction of greatest resistance. The molar teeth have been subjected 

 to much more severe direct irritation from use than any others in the 

 jaws, and this will account for their increased diameters. In the case 

 of the eutherian Mammalia, molar teeth are not traceable back to 

 ancestral types of reptilian molars, but to simple conic (haplodont) 

 reptilian teeth." 



" The greater number of types of mammals have derived the char- 

 acter of their molar teeth from the stages of the following succession : 



" (1.) A simple cone or reptilian crown, alternating with that of 

 the other jaw (haplodont type)." In the upper jaw the cone is termed 

 a protocone, in the lower jaw a protoconid. 



(2.) To the simple cone are first added three or four small, denticles 

 along the edges ; these are then replaced by an anterior and a posterior 

 cusp (Fig. 370, A), in the upper jaw termed paracone and metacone, 

 and in the lower jaw paraconid and metaconid respectively. Thus is 

 produced the triconodont type, exhibited by the Triconodontidse, early 

 Jurassic Mammals. " No mechanical cause can be assigned for the 

 development of these cusps from the denticles, but the nutrition of 

 the parts probably has had an important influence on the process. 

 Each basal cusplet stands nearer to the point of entrance to the crown 

 of the nutritive artery, which ascends or descends through the root, 

 than any other cusplet, and would therefore grow more rapidly than 

 any other secondary part of the crown under stimulus of use. The 

 basal cusplets have thus replaced those occupying more elevated 



