IOWA CITY LOESS. 209 



Planorbis exacutus Say. 



Locally common in shallow ponds. 



* Planorbis parvus Say. 



Very common in ponds, etc. Rare in the western loess. 



* Planorbis albus Muell. 



Quite rare in rather more permanent ponds. Prof. 

 Udden collected one specimen in the loess of Milan, 111. 



Planorbis dilatatus Gld.(l). 



Locally rather frequent. None fossil. 



*Segmentina armigera (Say) H. & A. Ad. 



Common in ponds. Reported from the loess of Missouri, 

 but not known as a fossil in Iowa and Nebraska. 



Ancylus diaphanus Hald. 



Common on sticks, etc. in more permanent ponds. 



*Valvata tricarinata (Say) . 



Quite common. It also occurs in deeper ponds. Prof. 

 Udden collected it in the loess of Milan, 111., and it is re- 

 ported from Missouri. It is, however, very rare as a 

 fossil. 



Pisidium compressum Prime (?) . 



A few valves of this, or a closely related species, were 

 found in the local loess. The species is now common, 

 especially westward, in small prairie streamlets, etc. 



All but the last two species in this list are air-breathing 

 forms. The last one is a bivalve. 



b. Species of deeper ponds, bayous, etc. 



GASTEROPODS. 



Bithyneixa OBTUSA (Lea) St. Quite common. 

 Amnico^a cincinnatensiS (Anth.). Very common. 

 Amnicoi,a limosa (Say) Hald. Quite common. 



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