230 COPE. (Vou. IIE. 
the teeth in the opposite jaw as comes in contact with them 
when in use. The development of the heel in the inferior 
molars produced a type which is known as the tuberculosecto- 
rial. This type characterizes the Creodonta and a few Carnivora. 
In the former there are generally three such teeth, in the latter 
but one. 
Figure 55.— Molars of Triassic and Jurassic Mammalia, enlarged; from Osborn: 
z, Dromotherium; 2, Microconodon; 3, Amphilestes; 4, Phascolotherium; 5, Tri- 
conodon; 6, Peralestes; 7, Spalacotherium; 8, Amphitherium; g, Peramus; zo, As- 
thenodon; zz, Dryolestes; 72, 73, Amblotherium; 74, Achyrodon; 75, Kurtodon; 
all inferior molars except Figs. 5, 6, and 15. 
6. ORIGIN OF THE TUBERCULOSECTORIAL MOLAR. 
In this type of inferior molar the primitive tritubercular part of 
the crown stands principally anterior to the posterior root of the 
tooth. It appears that the posterior root has been extended back- 
wards, so as to occupy a position below the middle of the superior 
molar, while the tritubercular crown has been confined to the 
space between the crowns of the superior molars. This would 
follow of necessity from the alternating action of the crowns of 
the opposite series, in connection with a general increase in 
