212 Natural History Bulletin. 



Linmoea humilis. 

 Heltcodiscus lineatus. 

 Pupa armifera. 

 Patula alternata. 

 Patiila strigosa cooperi. 

 Stenotrema leaii. 



Those which are placed in the left-hand column are more 

 abundant and more generally distributed, and ma\- be con- 

 sidered the characteristic fossils of the Loess of Iowa and 

 Nebraska. The first three particularly are excedingly abund- 

 ant, and the writer has yet to find an exposure of fossiliferous 

 Loess in which they do not occur. 



In the distribution of the fossils we find the same pecu- 

 liarities which ma}' be observed in that of the recent shells. 



Occasionally exposures are found in which a certain species 

 will occur in great numbers, but restricted to a very narrow 

 portion of the exposure, just as we find limited localities in 

 which the recent shells of the same species are very abundant, 

 largely to the exclusion of others. Such are notably Hdicina 

 occulta, Mesodoii imiltilmeata, the Limnace, etc. 



Others again are very generally and quite uniformly scat- 

 tered throughout large portions of the Losss, both horizontally 

 and vertically, while the occurence of still others is excep- 

 tional. The same species which to-day show the greatest 

 adaptability to circumstances, and which cover the widest 

 range in habitat are, as a rule, the most widely distributed in 

 the Loess. 



The study of the habitat of the species represented in the 

 Loess is of special interest. All of the species herein listed 

 are partial to moist, or even decidedly wet places, though 

 Patula trigosa cooperi. Pupa alticola and Succinea lincata (the 

 two latter among the most abundant of our fossils) are now 

 found in great numbers in regions with a dry or very dry 

 climate, though they are active only in moist places in these 

 regions. 



Of the remaining species, Helicina occulta and Vertigo 



