338 VERTEBRATE ZOOLOGY 



times had teeth in which the tubercles are arranged in a triangular 

 or trigon group, with the main cusp on the inner edge of the tooth. 

 These Trituberculata, as they are called, probably had insectivorous 

 habits and may have been the direct ancestors of the insectivores of 



FIG. 175. Jaw of Triconodont mammal, Triconodon ferox, Comanchian, 

 Wyoming. Three times natural size. (From Lull, after Marsh.) 



to-day. Still another early group, the Allotheria (Fig. 176), found as 

 early as the Jurassic, but lasting over into the Cenozoic, had multi- 

 tuberculate molars and rodent-like incisors. The premolars were in 

 some cases much like the primitive cutting teeth of the Trituberculata. 



All of these mammalian relics indicate 

 that the mammals made a very modest 

 and tentative start in the Mesozoic. If 

 one may judge by the teeth, 'they had 

 already undergone a limited adaptive 

 radiation into insectivorous, carnivorous, 

 FIG. 176. Skull of multi- anc j gnawing types, which foreshadowed 

 tuber culate mammal (allo- ., ,. ,., , , .. 



there) PtOodus gracilis, Pal- the mammalian groups of like habits to- 

 seocene. (Ft. Union) ; Wyom- day. The reason for their inconspicuous- 

 ing. About natural size. ness during the Mesozoic is not far to seek, 

 (From Lull, after Gidley.) r , ,. e , , ., , , 



for they lived when the reptiles had pre- 

 empted all of the important life ranges. Their very inconspicuous- 

 ness was their salvation and gave them a chance to live through a 

 trying period and to await the dawning of their great opportunity; 

 this came toward the end of the Cretaceous, when the reptilian 

 dynasties waned and extinction overtook all of the highly spe- 

 cialized dominant types. Perhaps, as has already been suggested, 

 these small, blood-thirsty mammals played an important role in 

 hastening the decline of the reptiles by preying on their eggs 

 and young. One may picture the Mesozoic drama in the words 

 of Lull, if we bring "before tlie mind's eye broad vistas of low- 



