18 



MAMMALIA 



as in most rodents, and the condyle moves in a longitudinal depression 



of the squamosal bone, the ridges are placed transversely to the longitudinal 



axis of the teeth. This arrangement is 

 found, also, in the Proboscidia, Avhere 

 the mandible moves antero-posteriorly 

 (palinal). A bunodont as well as a 

 lophodont tooth may become very 

 complicated, by a folding of the enamel- 

 ridges, by the formation of spur-like 

 processes, by the insertion of adjoining 

 and secondary cusps, by the development 

 of cingula, and finally by the addition of 

 cusps and ridges on the posterior sur- 

 face. As a rule, the upper cheek-teeth, 



which extend over those of the lower jaw, are broader than the latter ; these, 



however, become enlarged antero-posteriorly. 



It has been argued with much ingenuity by Cope and Osborn that the 



multitudinous modifications of the molar teeth have been derived from the single 



Fig. 20. 



Development of the molars of Ungulata. Upper 

 and lower molar.s superimposed, pr, pa, me, ml, 

 prd, iMd, med, end, hyd, as in Fig. 19 ; liy, hypocone ; 

 pi, protoconule. (After Osborn.) 



A, Elotherium, lower molar bunodont. B, Hyrachyus, lower molar lophodont. C, Anoplotherium, ectocones 

 crescent-shaped ; eudocoues conical, m-^ with a supernumerary crescent. 



■J^" 



cone characterising reptilian teeth. This primitive haplodont tooth gives rise 

 to the protodont type, as lateral cusps are formed and the root begins to divide. 

 When the lateral cusps approximate the principal cone in size, there may be 

 distinguished either the iriconodont tooth, with three cones in a row, or the 

 trituhercular, which has the a b c 



lateral cones displaced to 

 one side and alternating with 

 the principal cone. In both 

 types the root is divided. 



Triconodont teeth are 

 limited to but a few groups 

 of mammals, and are not of 

 especial significance for trac- 

 ing the evolutionary history 



of molars. Much more important for this purpose is the trituhercular type of 

 dentition, the earliest manifestations of which are perceived among certain 

 Cynodontia, the reptile-like precursors of mammals. 



In the trituhercular dentition the so-called frigons of the upper, and trigonids 

 of the lower, jaw were eftective organs so long as the mammals developing them 

 remained of small size and subsisted chiefly on insects. With the gradual 

 increase in size of body and change to a carnivorous or mixed diet, stronger 



Inferior aspect of four-, five-, and six-cusped upper molars. 

 A, Prodreinotherium. B, Anoplotherium. C, Lophiothenum. 



