122 B. SHIMEK AFTONIAN FOSSILIFEROUS GRAVEL AND SAND BEDS 



In number of individuals the aquatic species predominate, the land 

 shells being represented by but few specimens, which were evidently 

 washed from near-by land surfaces. 



These land species are all living in Iowa today, and their presence in 

 the Aftonian is especially suggestive, since land shells suffer much more 

 from climatic changes than do the aquatic species. Their presence proves 

 the proximity of plant-covered land areas similar to those which now pre- 

 vail in the region under discussion, and a climate not materially different 

 from that which is found in Iowa today, at least so far as temperature is 

 concerned. 



COMPARISON WITH MODERN M0LLU8E8 



In order that this similarity of Aftonian and modern moUuscan faunas 

 may be more fully appreciated, a comparison is here made between the 

 fossils of the foregoing list and a series of shells dredged in Miller Bay, 

 West Lake Okoboji, lowa,'^ during August, 1908. 



This bay forms a regular rounded indentation in the west shore of the 

 lake. The dredging was done in shallow water, 4 to 8 feet in depth, near 

 the head of the bay, just opposite the Iowa Lakeside Laboratory. 



All the species in the foregoing list which are marked with an asterisk 

 were also dredged from Miller Bay. Where only generic names are given, 

 the Aftonian specimens are more or less imperfect, but apparently iden- 

 tical with species collected in the bay. 



Unio anodontoides Lea {= Lampsilis anodontoides (Lea) Baker) and 

 Unio metanevrus Eaf. {=Quadrula metanevra (Eaf.) Simp.) of the 

 Aftonian seem to find substitutes in Unio luteola Lam. {= Lampsilis 



luteolus (Lam.) Baker) and Anodonta of the bay; Valvata hicarinata 



is found living in other parts of Iowa today; Ancylus rivularis is now 

 common in the Des Moines Eiver, less than 20 miles away, and Lymncea 

 reflexa is common in ponds in the vicinity of the lake and on other parts 

 of the lake shore. 



The bay also yielded 4 additional species of Planorbis, 2 of Lymncea 

 and 1 of Bulinus, all these aquatic and evidently living in the bay or in 

 ponds communicating with it. The similarity of the aquatic fauna of the 

 bay to the aquatic fauna of the Aftonian is striking. It is a fauna belong- 

 ing to the larger streams and lakes, and is unlike the terrestrial fauna 

 with its occasional small-pond pulmonates which characterizes the loess. 



It will also be noticed that three of the terrestrial species occurring in 

 the Aftonian were also dredged from the bay. Two specimens of Succinea 



■^ Lake Okoboji Is In northwestern Iowa, and the terrestrial molluscan fauna of its 

 vicinity is practically the same as that of the region of the Aftonian exposures herein 

 discussed. 



