POST-WOODFORDIAN MOLLUSKS 27 



populations were not abundant at any time in the history of the Strawn NE and 

 Batavia W bogs, since population density per cubic unit is essentially constant 

 except in the lowermost few inches of the deposits. 



3 . Although there is evidence of higher water levels in the basins that 

 now contain the bogs and bog lakes at Strawn NE and at Batavia W, there is 

 also some evidence that water levels fluctuated widely. Physa gyrina 

 hildrethiana is recognized as especially adapted to surviving ephemeral pools, 

 and as a result seldom reaches full size. The almost uniformly small size of the 

 examples recovered tends to support this thesis, although obligate aquatic mol- 

 lusks occur in the same samples. The conclusion is that P. g. hildrethiana 

 lived in peripheral parts of the lake, and were left stranded as water levels fell, 

 while amnicolids and other branchiates lived in the open waters at the center of 

 the bog basin. 



4. The sparsity of specific data concerning the ecological requirements 

 of freshwater mollusks makes it difficult to describe the conditions under which 

 the mollusks lived in the Strawn NE and Batavia W bogs. The most that can be 

 done is to use the time-honored (but not necessarily accurate) system of com- 

 paring the fossil assemblages with living molluscan faunas. On this basis, it 

 can be concluded that the molluscan faunal assemblages in the two bog lakes 

 lived in relatively clear, cool waters, moderately eutrophic, basic in reaction, 

 sufficiently supplied with dissolved carbonates to support strong growth of mol- 

 luscan shells, and characterized by fluctuating levels. This favorable environ- 

 ment drew to a close about 2,000 radiocarbon years B.P. when, apparently 

 because of declining water levels, sphagnum moss produced a matte that prob- 

 ably covered the entire surface of the open water. Molluscan populations 

 declined sharply at this time; shells are found only in the lowermost 2 to 3 inches 

 of the peat. Molluscan populations in the two lakes are now very sparse. 



5. No evidence of the Altithermal Interval at about 7,500 radiocarbon 

 years B.P., which should occur at approximately the 10-foot level in the two 

 bogs, can be seen in the faunal assemblages. 



Finally, as far as is known to me, the molluscan faunal assemblages 

 reported here are the first freshwater faunas reported from the time interval 

 10,000 to 2,000 radiocarbon years B.P. and documented by direct radiocarbon 

 analyses. 



Core samples would add to the accuracy of the knowledge of the vertical 

 distribution of mollusks in these sediments; and studies of diatoms, ostracods, 

 and pollen would broaden the scope of an understanding of the post-Woodfordian 

 geological history. 



REFERENCES 



Baker, P. C, 191°, The ecology of the Skokie Marsh area, with special reference to the 



mollusca: Bull. 111. State Lab. Nat. Hist., v. 8, p. 441-499. 

 Baker, F. C, 1911, Lymnaeidae of North and Middle America, recent and fossil: Chicago Acad. 



Sci., Spec. Publ. No. 3, p. i-xvi, 1-539, 58 pis., 51 figs, in text. 

 Baker, F. C, 1918, Post-glacial mollusca from the marls of central Illinois: Jour. Geology, 



v. 26, p. 659-671. 

 Baker, F. C, 1928, The freshwater mollusca of Wisconsin: Wisconsin Geol. Nat. Hist. Surv. Bull 



70, pt. 1 (Gastropoda), 494 p., 28 pis., 202 figs, in text; pt. 2 (Pelecypoda ) , 482 p., 



105 pis., 294 figs, in text. 

 Baker, F. C, 1930, A new record of Gastoroides ohioensis from Illinois: Science, v. 71, p. 389- 



