THE DISTRIBUTION OF LOESS FOSSILS 125 



fewer and more scattered specimens on hillsides, etc., especially 

 in more open places. To get a good set of any species in such 

 localities the collector must work over a considerable area, but in 

 doing so he will almost invariably find individuals of several species 

 mingled promiscuously. If he compares the molluscan faunas of 

 the eastern and western parts of the state, he will find that, as 

 stated, the number of species and individuals in the eastern part 

 is, as a rule, greater. He will also find that there are certain rather 

 striking differences between sets of some of the species taken at 

 opposite extremities 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 Polygyra mutilineata, Zonitoides minus- 

 culits, Siiccinca 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 mol- 

 luscs, 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 abun- 

 dance 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 



