42 Wortman — Studies of Eocene Mammalia in the 



in Dissacus, is to be accounted for on the ground of a better 

 adaptation to the modified structure of the superior molars, 

 which gradually supervened in the course of their develop- 

 ment. 



The molars and premolars of the superior series exhibit 

 some very distinctive modifications, and as these are in such 



63 



Figure 63. — Left upper teeth of Dissacus saurognatlius Wortman; crown 

 view; one and one-eighth natural size. (After Osborn.) 



Figure 64. — Left upper teeth of Mesonyx obtusidens Cope ; crown view ; 

 three-fourths natural size. (After Scott.) 



strict accord with what we know has taken place in many 

 other well-established phyletic series wherein greater complexity 

 was attained, we may safely assume that the later condition in 

 this group is the more advanced, specialized, or progressive. 

 In Dissacus, figure 63, the first two premolars have simple 

 premolariform crowns, with small posterior heels. The third 

 is of much interest, in that it exhibits a transition from the 

 simple type of tooth crown to the more complex crown of the 

 molars. It is of the utmost importance to note that the prin- 

 cipal, and, therefore, the primitive cone, or cusp, of the 

 simple stage is located as usual about the middle of the tooth 

 crown, and that the heel has been developed into a small but 

 distinct cusp immediately posterior to it. At the same time 

 an internal cusp has appeared, just internal to, or on the 

 lingual side of, the principal or primitive cusp. The crown of 

 the fourth premolar exhibits the same elements in exactly the 

 same order of arrangement, but somewhat more advanced to- 

 wards the true molar type. The main cusp has its position 

 apparently changed to a* slightly more external one, by reason 

 of the increase in size of the posterior external cusp and the 

 spreading out and enlargement of the internal cusp. A 



