336 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



recent Mammalia the carnivorous and insectivorous 

 Marsupialia generally have the tritubercular lower mo- 

 lar with heel. In the Chiroptera and many Insectivora 

 the heel is largely developed, and supports two cusps, 

 as it does in some Creodonta. 



- 99- Deltatherium fundaminis Cope, fragmentary skull, two-thirds 

 natural size ; from the Puerco bed of New Mexico. a,b, c, from one individ- 

 ual ; d, from a second animal; a, right side of cranium; b, palate from be- 

 low ; c, mandible, part from above ; d, left ramus, outer side ; from the Re- 

 port of the U. S. Geol. Surv. Terrs., Vol. III. 



From this point the evolution of the tritubercular 

 molar must be considered from two standpoints. The 

 first is the mechanical cause of the changes of its form ; 

 and the second is the mechanical cause of its definite 



