OF THE MESOZOIC MAMMALIA. 225 
alternatives are to place them in three families or in one. I have finally adopted the 
latter^ upon grounds which are more fully stated later. 
In describing the teeth a distinction has been made between a “heel” or sloping 
extension at the base of the crown, a “ basal cusp” and a “ cingulum cusp,” which 
is simply a prominent portion of the cingulum. These distinctions also enter into 
the classification frequently, especially in connection with the premolars. 
The chief molar types of this group are shown in the accompanying figure. 
Figure 10. The principal molar tooth forms of the Mesozoic Mammals of the Second Group. The 
anterior face of the molars throughout is to the left, and the posterior face to the right. A, Dromothe- 
rium, the second lower molar, inner face X 7. B, Microconodon, the fourth lower molar, outer face X 7- 
1, AmphiUstes, the second lower molar, inner face. 3, Phascolotherium, the fifth lower molar, inner face 
4, Triconodon, the second lower molar, inner face. 6, Peramus, the fourth lower molar, outer face. 7, Spala- 
colherium, the third lower molar, inner face ; a, outer face. 8, Peralestes, third upper molar, inner face. 
9, Peraspalax, third lower molar, inner face. 10, Leptocladus, third lower molar, outer face. 11, Phascolestes, 
third lower molar, inner face. 13, Achyrodon, fourth lower molar, outer face. 12, Dryoleates, lower molar, 
inner face ; a, outer face ; b, wearing surface. 15, Kurtodon, upper molar; a, wearing surface. Original. 
In the first type there are three cusps in the same fore-and-aft line (figs. 1, 3, 4, 11, 
13). In the second the cusps are placed upon opposite sides of the crown and 
separated by a median valley (figs. 8 and 9) . In the third the cusps are placed upon 
opposite sides of the crown, but connected by transverse ridges (figs. 12, 12a and b). 
In the fourth the crown is columnar, there are no cusps, and the fangs are placed 
transversely (figs. 15 and 15a). Transitional types are seen in figs. 6 and 7. Fig. 10 
represents an isolated type. The next figure shows the form of the premolars 
which accompany these types, the numbers corresponding to the above. A compari- 
^ In the preliminary abstract of this -paper Amphilestes was embraced in the Triconodontidx, and Phas- 
colotherium and Spalacotherium made the types of other families. 
