44 EVOLUTION OF MAMMALIAN MOLAR TEETH 



triangles the jaws remained nearly isognathous (Fig. 36, Xo. 4). There 

 is no evidence as to the origin of the hypoconid, which as a rule preceded 

 the hypocone, as it was developed very early. In the Stylacodontidse 

 (Figs, 22-35), PJiascolestes, Amblotherium, etc., the crowns rapidly increased 

 in transverse diameter, and, in some genera (Kurtodon, Fig. 13) they so 

 i';ir lost the tritubercular aspect that, but for the connecting form 

 Asthenodon (Fig. 35), 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 buno- 

 dont molars the primitive triangles retain their primitive alternating 

 arrangement. Second, the jaws are somewhat anisognathous. Third, in 

 support of the first law of cusp development, we observe that the proto- 

 comile and metaconule are developed at the points of contact with the 



PIG. 37. Diagram of quadritubercular molars of both jaws iu normal mutual relation ; the 

 superior molars in double lines ; the inferior in black. 



ridges which extend from the hypoconid, and, secondly, that the hypocone 

 appears at the point where the paraconid abuts against the hypocone. It 

 follows from a comparison of numerous species of Pelycodus and Miocluenus 

 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, the upper molars develop 

 into the sexitubercular, the lower into the quadritubercular type. 



The complemental development of the upper and lower molars in the 

 known genera of successive horizons is approximately displayed in the 

 subjoined table. The Eocene list of genera will be greatly reduced, 

 especially in the tritubercular-sectorial type, when the upper and lower 

 jaws are found associated, and it must be clearly understood that the sub- 

 types o, b, c, in this table, are closely related by transition forms. In 

 fact, in carnivorous forms, the extreme secodont and bunodont types are 

 frequently seen side by side, as in the first and second inferior molars of 

 Didymictis. The chief distinction between these two series is the greater 

 development of the secondary cusps and the almost invariable loss of the 

 paraconid in the latter; this is effected by the broader surfaces of contact 

 in the bunodont crowns. In the secodont series, on the other hand, the 



