IOWA ACADEMY OF SCIENCES. 101 



will also find that there are certain rather striking differences 

 between sets of some of the species taken at opposite extremi- 

 ties of the state. Those from the eastern part are likely to 

 average larger in size and to be thinner-shelled, resembling 

 more nearly representatives from the eastern part of the 

 country, while the western forms are smaller and heavier. 

 This is especially true of Polugijra multlUneata, Zonitoides minus- 

 culus, Succinea obliqua, S. avara, and other species of the kind 

 which are sometimes found in rather low places, but which also 

 occur on higher grounds, especially westward. This is prob- 

 ably due chiefly to the scarcity of forests in the western and 

 central parts of the state, where the rather scant groves usually 

 consist of scattered and stunted trees, being quite different 

 from the more vigorous forests of the eastern part. That this 

 view is correct, is further attested by the fact that the same 

 species of molluscs, when occurring on comparatively barren 

 or nearly treeless areas in the eastern part of the state, usually 

 show the characters of the western types— namely, the smaller 

 size and sometimes heavier, or at least more compact, shell. 



If the student will study the molluscs of a given region for 

 a number of years, he will find that from year to year the 

 abundance of the several species varies, some even running 

 out entirely, while others unexpectedly appear. The writer 

 has watched a number of localities near Iowa City for many 

 years, and has found this variation often striking. 



If, now, the distribution of the fossils in our loess is com- 

 pared with that of the modern shells, a remarkable similarity 

 is evident. The best collecting grounds are near streams, 

 while the clay of the remote prairie is usually barren. Where 

 fossils are abundant, one exposure contains species of one kind, 

 another near by presents a new, or at least a different list, while 

 still another has none, and the same variation which may be 

 observed in the local distribution of the recent shells in any 

 restricted locality, will be exhibited in individual exposures of 

 fossiliferous loess. 



In horizontal distribution the fossils show the same mode of 

 distribution as that already noted in the modern forms. The 

 specimens are not heaped together, but are scattered about 

 like the modern shells, usually a number of species mingled 

 together, but in unmodified loess invariably not crowded, so far 

 as the writer's experience goes. 



The vertical distribution of the fossils also conforms to the 



