DENTAL SYSTEM 



3> 



which the teeth were in the form of almost a quite simple cone ; 

 such a presumably primitive type of dentition being apparently 

 retained among some existing Edentates, like the Armadillos, while 

 it is possible that we should regard the dentition of the existing 

 Cetacea (Fig. 2) as a reversion to the same primitive type. None of 

 the Mesozoic mammals at present known exhibit this simple 

 conical type of teeth, although we have an approximation to it in 

 the extremely generalised genus Dromatherium. Starting then 



FIG. 4. Molar teeth of Mesozoic Mammals (enlarged). Triconodont type 1, Dromatherium ; 

 2, Microconodon ; 3, Amphilestes ; 4, Phascolotherium ; 5, Triconodon. Tritubercular type 6, 7, 

 Spalacotkerium ; 10, Asthenodon. Tubercular sectorial type 8, Amphitherium ; 9, Peramiw; 11- 

 13, Amblotherium ; 14 (?) Amblotherium. pr, Protocoue ; hy, hypocone ; pa, paracone ; me, 

 metacone, in the upper teeth ; and protoconid, hypoconid, paraconid, and metaeonid in the 

 lower. 6 and 15 are upper molars, and the rest lower molars. (After Osborn.) 



from this presumed simple cone it appears that the teeth of Dromu- 

 therium (Fig. 4, i) present the first stage towards trituberculism, the 

 crown of each tooth having one main cone, with minute lateral 

 cusps, and the root being grooved. In the next or true Tricon- 

 odont stage (Fig. 4, 3-5) the crown has become elongated antero- 

 posteriorly, and consists of one central and two lateral cones or 

 cusps, while the root is divided. From this the transition is easy to 

 the tritubercular type, in which the three cusps, instead of being 

 placed in a line, are arranged in a triangle ; the upper teeth (Fig. 



