Evolution of Mammalian Molars. 1073 
and in the Tertiary period, by Professor Cope, so that only a brief 
résumé is necessary here. In Dromotheriwm (fig. 1), from the upper 
Triassic, the oldest mammalian type known, with the exception 
ane 1.—Diagram of quadritubercular molars of both jaws in normal mutual rela- 
on; the superior cusps double lines ; the inferior black. 
of Microlestes, the molars have a main protoconid with several 
minute lateral cuspules, differing in size in the different teeth, but 
in general giving a trifid appearance to the crown. The molars of 
the contemporary Microconodon (fig. 2) also have unpaired fangs, 
but distinctly trifid crowns, with the anterior and the posterior 
cusps, or para and metaconids, upon the slopes of the protoconid. 
This Triconodont type reappears, with the addition of a cingulum 
and paired fangs, in Amphilestes (fig. 3) and Phascolotherium (fig. 4) 
of the lower Jurassic and persists in Tricondon (fig. 5) of the upper 
Jurassic. In this succession we observe especially the relative 
subsidence of the protoconid and upgrowth of the para- and 
metaconids. Contemporary with -Amphilestes is the classical 
genus Amphitheriwm (fig. 6). A recent examination of the type 
Specimen by the writer revealed the very interesting fact that 
the molars of, this genus are probably of the primitive tubercul- 
_ ar- sectorial types,—the oldest known example. Only the para- 
cone and metaconids and hypoconids have been observed heretofore, 
but one can see the tip of the main external cusps between the 
former pair. This pattern is repeated, with a considerable elevation 
of the heel, in Peramus of the upper Jurassic.’ Neither 
of the two foregoing are of the primitive heelless tritubercular type 
which is apparently found in Spalacotherium also upper 
Jurassic, and in the nearly related if not synonymous Peralestes, 
Plate XXV. Contemporary with the above, are numerous 
genera of the Stylodon order; among these, Asthenodon is of 
' This genus includes also Septocladus dubius Owen, and Spalaco- 
therium minus Owen. See pe Phila. Acad., Nov. 1888, p. 292. 
