THE MOLLUSCAN FAUNA OF McGREGOR, IOWA.* 
FRANK CoLuINs BakER. 
The 1904 Field Day of the Chicago Academy of Sciences 
was held at McGregor, Iowa, where several members of the 
Academy spent a profitable week during the month of July 
studying the Fauna, Flora and Geology of this interesting 
region. The ecology of the region is of more than usual 
interest. Here are high bluffs more than two hundred feet 
in height and several miles apart, between which flows the 
great Mississippi River, whose broad expanse of water, here 
almost a mile in width; rolls irresistibly on its journey to the 
Gulf. The river is dotted with islands covered with foliage 
and many of these islands have long sand or mud bars on the 
lower ends which afford an inviting refuge for many mollusks, 
particularly the Unionidae. Opposite South McGregor, which 
is the locality more particularly studied, there are several 
islands which have formed a perfect atoll, the narrow inlet 
and wider interior basin being faithfully represented. Such 
a station is especially adapted to the growth of mud-loving 
mollusks, such as Anodonta, Vivipara and some thin-shelled 
Lampsilis. : 
The geology of the neighborhood of South McGregor is 
notably interesting. The bluffs are made up of sandstone, 
limestone and shale, the strata belonging to: the Ordovician 
Age and including the Lower Magnesian Limestone, St. 
Peter’s Sandstone, Trenton Limestone, Galena Limestone 
and, farther inland, the Marquoketa shales. 
Land shells seemed unaccountably scarce, the abundance 
of limestone and deciduous trees affording, apparently, desir- 
* Presented by title to The Academy of Science, April 17, 1905. 
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