EVIDENCE BEARING ON TOOTH-CUS1' DEVELOPMENT 97 



much rounded and deeply divided on the inner or lingual side. 

 Thus, there is not the slightest suggestion of a tendency toward 

 an outward movement of the lateral pair of cusps, while it is 

 easily conceivable that the continued development of the two 

 inner heel cusps and outer cingula w r ould early result in a gen- 

 eral form of tooth very different in pattern from the tritubercular 

 type which might form the basis for such molars as those of the 

 diprotodont marsupials and many of the rodents or even of the 

 manatee and mastodon. I do not wish to be understood here 

 as implying any relationship between these very diverse forms, 

 but as especially emphasizing the fact that in Triconodon is sug- 

 gested an easy and not improbable way in which some complex 

 molars may have been derived without having passed through 

 the typical tritubercular stage. 



Thus, it is shown by this restudy of the two forms, which 

 according to Osborn represent successive stages in the evolution 

 of the mammalian molar, that the gap between them, which 

 was already great, even according to Osborn's interpretation, is 

 very greatly increased especially from the tritubercular theory 

 standpoint. Moreover there is no evidence, in the way of in- 

 termediate forms, indicating that Dryolestes ever passed through 

 a stage strictly analogous to that of Triconodon or that the 

 main internal cusp is in any way homologous to the central 

 cone in the Triconodon molars. Furthermore, a critical com- 

 parison of these two forms shows that such an hypothesis is beset 

 by many difficulties. The following are the principal ones : 



(1) The molars of Triconodon are larger and fewer in number 

 than in Dryolestes indicating a generally higher specialization. 



(2) The lateral cones in Triconodon are already comparatively 

 much specialized, being suplemented by growths of the cingu- 

 lum externally and heel cusps internally and thus do not es- 

 pecially resemble, either in form or proportions, any two of the 

 external cusps in Dryolestes. (3) The external portion of the 

 upper molar in Dryolestes (see PI. V, figs. 2 and 3) is composed 

 of three simple connate cusps supported by two fangs, their 

 general appearance suggesting an arrangement homologous to 

 the three cusps and two fangs of Triconodon; while (4) the 

 internal portion of the tooth is a high antero-posteriorly com- 



