356 S. CALVIN AFTONIAN MAMMALIAN FAUNA 



wide across the second ridge. The right tooth is slightly smaller, 8% 

 inches long, and the corresponding widths are also somewhat less. 



Leidy described the last lower molars of this species, 18 and he found a 

 somewhat similar disproportion between the right and the left. As in 

 the teeth described by Leidy, the main body of the Akron molars is made 

 np of six divisions or ridges. The sixth division, however, is somewhat 

 irregular, and in the right molar is made np of more than two lobes. Be- 

 hind the sixth ridge are elements of rudimentary ridges in the shape of 

 a number of mammiform tubercles, not shown in the specimen illustrated 

 by Leidy on plate xxv, figure 2, of the work cited. The wearing down 

 of the lobes of the transverse ridges exposes "large tracts of dentine bor- 

 dered by thick festooned bands of enamel." Our teeth are worn more 

 than the lower teeth described by Leidy, and in the third division, as well 

 as in the first and second, the dentinal tract is continuous. In the fourth 

 and fifth ridges the tracts are still separated by the thick enamel. 



The beds on the Loup river, from which the original specimens of Mas- 

 todon mirificus came, have been variously correlated with the Miocene, 

 the middle and later Pliocene, and the early Pleistocene. Osborn in U. S. 

 Geological Survey Bulletin 361, page 84, includes Mastodon mirificus 

 in the fauna of the Elephas imperator zone, but remarks later on the 

 same page that "the exact position of the Elephas imperator zone, also 

 the question of whether it is of the same age as the Equus zone, remain 

 to be determined." It is now certain that typical representatives of the 

 Equus fauna are associated with Elephas imperator and Mastodon mirifi- 

 cus in the Aftonian beds of western Iowa, and it is worthy of note that 

 Mastodon americanus occurs abundantly in the same association. Signifi- 

 cant among the new finds not heretofore recorded are a characteristic 

 tooth of the imperial elephant and a tusk and sixth molar of the Ameri- 

 can mastodon, which were taken from gravels under Kansan drift, in 

 wells near Mapleton. 



Mastodon mirificus Leidy may be compared with Cope's Dibelodon 

 liumboldtii Cuvier, referred to in the Fourth Annual Eeport of the Geo- 

 logical Survey of Texas. Cope regards the horizon from which his speci- 

 mens came as "more or less exactly equivalent to the Equus beds." 



18 Leidy : The extinct mammalian fauna of Dakota and Nebraska, Philadelphia, 1869, 

 p. 249, pi. xxv, figs. 1, 2. 



