34 IOWA ACADEMY OF SCIENCES. 



paper contains the names of the species which have been 

 authentically reported from the loess, or which are in the 

 author 's private collection. The numbers opposite the names 

 show the number of cleaned specimens in the collection which 

 are available for immediate study. Only those from the loess 

 of Iowa and Nebraska are there included, and they are divided 

 into two groups, — those from the eastern loess (the territory 

 including the counties of Des Moines, Muscatine, Scott, 

 Dubuque, Bremer, Johnson, Iowa, Polk and Warren in Iowa), 

 and those from the western loess (from the counties of Fremont, 

 Pottawattamie and Woodbury in Iowa, and Otoe, Sarjiy, Cass, 

 Douglass, Lancaster, Saunders and Gumming in Nebraska). 

 The numbers are given for the puri30se of showing the relative 

 number of specimens of the various species which have accu- 

 mulated in the collection in nearly a score of years, and which 

 form the basis for this paper. Of course this does not repre- 

 sent the entire number collected, for many have been sent out 

 in exchange. Neither does the table show the exact ratio of 

 the fossils in the loess, for of the common terrestrial forms 

 many were observed but not collected, whereas of the rarel- 

 forms and of the Limncpce nearly all which were observed were 

 taken, and few or none were sent out in exchange. The fossils 

 were compared with many thousands of modern sjDecimens. 



1. Aquatic or semi-aquatic MoUusca. 



The forms which may properly be included under this head 

 are the species of Limnoea Physa, Bulinus, Planorbis and Segmen- 

 tina, all of which are, however, pulmonates — and Valvata, Pisid- 

 ium and Unio, which are branchiate, strictly aquatic species." 

 Special stress has been placed upon these forms as proving the 

 presence of large bodies of water during the deposition of the 

 loess. The weight of their testimony is here considered in 

 detail. 



Liinncea. — It will be observed in Table I that there are at 

 present in the author's collection 771 fresh water specimens, 

 of which 750 belong to the pulmonate genus Limncea. Of the 

 latter number over 300 specimens were collected near Iowa 

 City in a "pocket," the exposed cross-section of which does 

 not exceed four square feet in area. The remaining specimens 

 were also found in restricted areas (as though deposited at the 

 edge of a pool or pond), — never being so generally diffused 



*SwaUow's Amnicola lapidaria from Missouri (see Table I) Is PomatiopsUi lapidaria 

 a terrestrial species. 



