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. 



Fig- 99- Deltatheriitm 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 V. S. Geol. Si<rv. 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 



