84 IOWA ACADEMY OF SCIENCES. 



That such bodies of water existed, though, as before stated, not 

 of the extent required by the lacustrine theory, is also shown 

 by the distribution of the pond mollusca, which are found in 

 bands or layers similar to those which may be observed on the 

 edges of our small ponds to-day. These layers are usually of 

 but slight vertical extent, showing that the ponds did not per- 

 sist during the entire period of deposition of the loess, but, like 

 the ponds of to-day, were subject to changes. But if the water 

 area was not great, comparatively little of the material carried 

 . by the winds could be deposited in this manner, and as a matter 

 of fact we find comparatively little loess which shows such 

 origin. 



Secondary loess, which had been subsequently eroded and 

 re-deposited on lower lands by running waters, and which 

 usually shows stratification, should not, of course, be consid- 

 ered in this conection. 



In the consideration of any theory of the mode of deposition 

 of the loess, two propositions, which seem to be capable of sat- 

 isfactory demonstration, should be borne in mind, namely, that 

 the loess was deposited under climatic conditions essentially 

 the same as those which prevail in the same region today; and 

 that the deposition was slow and continued through a period of 

 considerable extent. 



That the first of these propositions is true is shown by the 

 molluscs which furnish the most satisfactory evidence of the 

 character of the conditions supporting life during that period. 

 The same species, with but very few exceptions, which occur 

 in the loess, exist in abundance now throughout the region 

 under consideration, the distribution of the fossils being exactly 

 such as may be observed under present conditions. If, for 

 instance, we compare the modern molluscan fauna of eastern 

 Iowa with that of eastern Nebraska, we find certain differences 

 which are almost exactly duplicated in the loess faunas of the 

 two regions.' 



For instance, Succinea lineata W. G. B., the common suc- 

 cinea of eastern Nebraska, is also the most common succinea of 

 the loess of that region, whereas Succinea avara Say, the most 

 common succinea of eastern Iowa, is also the most common 

 species of the genus in the loess of the same region. 



The majority of our species show a like distribution,^ plainly 



' No reference is here made to the Lamellibranch and Prosobranch fluviatile f auuas, 

 which seem to have spread into the region in question from their center of distribu- 

 tion in the southeast comparatively recently. 



sThe loess fossils of Europe are likewise like the modern forms inhabiting the 

 same region. 



