IOWA CITY LOESS. 207 



These shells represent more than 90% of the fossils in 

 the loess, and, the fossil fauna so far as it occurs, is very 

 similar to the living surface fauna. The latter is richer in 

 species, but this difference may be only apparent. A close 

 study of the local living fauna shows that species 

 frequently occur in very restricted areas. Different parts 

 of the same slope, often but a few feet apart, not in- 

 frequently show much variation in the distribution of 

 species. When we take this peculiarity of distribution of 

 land-shells into account with the comparatively very small 

 total area of all the loess exposures of this vicinity, we can 

 readily see that the opportunities for finding the recent 

 shells are much better. 



While, as stated, the great majority of the local loess 

 fossils belong to the preceding list of terrestrial forms, a 

 few additional species of aquatic habit are also sparingly 

 found. In no case, however, are they generally distribu- 

 ted in the loess, and with the possible exception of 

 Limncea caperata and L. humzlis, very few individuals are 

 found. In order that the comparative scantiness of the 

 fossil aquatic molluscan fauna may be more fully appreci- 

 ated a list of the local aquatic species is here given. Those 

 which also occur in the local loess are marked by two 

 asterisks, and those which are positively known as fossils 

 only from other localities are marked by one asterisk. 



2. Aquatic species now living at Iowa City. 

 a. Species of smaller ponds, etc., which often 

 become dry in summer. 



*Limn^EA REFLEXA Say. 



Common some years in shallow ponds. Not found in the 

 loess of Iowa and Nebraska, but reported from Missouri by 

 Hambach.* 

 **Limn.ea caperata Say. 



Locally very common. As a fossil it occurs in but few 

 * Geol. Sur. of Mo., Bull. no. 1, p. 82; April., 1890. 



