No. 2.] THE HARD :-PARTS OF THE MAMMALIA. 229 
differs from the primitive type in having a well-developed heel. 
Among recent Mammalia the carnivorous and insectivorous 
Marsupialia generally have the tritubercular lower molar with 
heel. In the Chiroptera and many Insectivora the heel is largely 
developed, and supports two cusps, as it does in some Creodonta. 
Figure 54.— Deltatherium fundaminis Cope, fragmentary skull; two-thirds 
nat. size; from the Puerco bed of New Mexico. Figs. a, 4, ¢ from one individual; 
Fig. d, from a second animal; Fig. a, right side of cranium; 4, palate from below; 
c, mandible, part from above; 4d, left ramus, outer side; from the Report 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 mechani- 
cal cause of the changes of its form; and the second is the 
mechanical cause of its definite location in a particular part of 
the jaw. For it has been already stated that in the evolution of 
the sectorial dentition of the Carnivora, the number of molars 
and premolars has considerably diminished. 
In the tritubercular dentition the crowns proper of one jaw 
alternate with those of the other (Fig. 56); but when heels are 
added in either jaw, they will oppose such part of the crowns of 
