1076 Evoiution of Mammalian Molars. 
therium, ete., the crowns rapidly increased in transverse diameter 
(fig. 7) and, in some genera, they (Kurtodon) so far lost the tritu- 
bercular aspect that, but for the connecting form Asthenodon (fig. 
6), we might hesitate to place them in this series. The key to the 
further evolution of the crown is seen in the bunodont series during 
the lower Eocene period. 
The superposition of the lower and upper molar patterns brings 
out many interesting facts. First, even in the complex crowns of 
the bunodont molars the primitive triangles retain their primitive 
alternating arrangement. Second, the jaws are somewhat anisog- 
nathous. Third, in support of the first law of cusp development, 
we observe that the protoconule and metaconule are developed at 
the points of contact with the ridges which extend from the hypo- 
conid, and, secondly, that the hypocone appears at the point where 
the paraconid abuts against the protocone. It follows from a com- - 
parison of numerous species of Pelycodus and Miocloenus that as the 
hypocone develops, the paraconid recedes, as first observed by Cope; 
a fact difficult to reconcile with the kinetogenesis theory. In this 
manner the inferior primitive triangle is broken, as the upper 
molars develop into the sextubercular and the lower into the quad- 
ritubercular type. The complemental development of the upper 
and lower molars in the known genera of successive horizons 18 
approximately displayed in the subjoined table. i 
he Eocene list of genera will be greatly reduced, especially m 
the Tritub.-tuberc-.sectorial type, when the upper and lower Jaws 
are found associated, and it must be clearly understood that the 
sub-types a, b,c, in the above table, are very closely related by 
transition forms. In fact, in the carnivorous forms, the extreme 
secodont and bunodont types are frequently seen side by side, as 0 
the first and second inferior molars of Didymictis. The chief dis- 
tinction between these two series is the greater development of the 
secondary cusps and the almost invariable loss of the paraconid y 
the latter ; this is effected by the broader surfaces of contact in the 
bunodont crowns. In the secodont series, on the other hand, the 
development of the secondary cusps is subordinated, and th 
conid is almost invariably suppressed.’ 
1 See Cope: “ Origin of the specialized Teeth of the Carnivo 
Naturalist, March, 1879. 
e meta- 
ra.” Am’ J 
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