THE MOLLUSCAN FAUNA OF McGREGOR, IOWA.* 



Frank Collins 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 aud 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- 



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CD * Presented by title to The Academy of Science, April 17, 1905. 

 CD 



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