212 NATURAL HISTORY BULLETIN. 



it will be further noticed that even these aquatic fossils 

 belong to the fauna of the small pond or streamlet which 

 may, and often does, remain dry during the greater part of 

 summer, and that their presence in no wise proves that 

 large bodies of water existed where the loess was deposited. 

 Indeed the total absence of species which are truly 

 fluviatile, or which at least prefer larger bodies of water, 

 would point to the contrary conclusion. These fluviatile 

 species are today very abundant in this vicinity, and their 

 shells are, for the most part at least, quite heavy, and 

 often large. Had large streams or other bodies of water 

 existed where the loess is deposited, thus furnishing con- 

 ditions favorable to this fluviatile fauna, it is reasonable to 

 suppose that some of these shells would be found fossil 

 today to relate the story of the conditions under which 

 they existed. Yet no such evidence has ever been found in 

 undoubted undisturbed loess, and the conclusion that such 

 bodies of water did not exist where loess is now found is 

 irresistible. Indeed the molluscan fauna of the loess points 

 to comparatively dry, upland, terrestial conditions such as 

 exist over the greater part of Iowa today. It suggests 

 land-surfaces clothed with vegetation offering shelter and 

 food to terrestial snails, — a vegetation developed under 

 medium conditions of moisture and temperature such as 

 exist here todav. 



