POST-WOODFORDIAN MOLLUSKS 25 



materials, including diatoms in the lowermost sediments, and increasing quan- 

 tities of plant remains as the lakes increased in age, point to a condition of 

 eutrophy. " Mollusks are most plentiful in eutrophic lakes with hard water, less 

 common in oligotrophic lakes, and absent from certain dystrophic lakes with 

 little calcium" (Hunter, 1964, p. 87). Inasmuch as the two bog lakes were 

 apparently eutrophic in a strongly basic environment since marl formed in large 

 quantitites, it is not surprising that populations of many species of mollusks 

 were maintained there for a long period of time (approximately 10,000 radio- 

 carbon years) . 



Several studies of bog lakes or ancient bog deposits have been made in 

 the general latitude of northeastern Illinois, one of the most interesting and 

 informative of these being an investigation of Dollar Lake, a few miles northeast 

 of Kent, Portage County, Ohio (Dexter, 1950). Dexter attempted to correlate 

 molluscan faunal assemblages with vascular plant zones and with pH. He found 

 that at depths greater than 12 feet, the bottom was bare of vascular plants and 

 devoid of mollusks. He also found that in spite of the general basic condition 

 of a bog, surface waters reached a pH of 6 . at the time of the spring turnover. 

 Unfortunately, Dexter makes no mention of precise quantities of dissolved car- 

 bonates in the water. He reported the occurrence of 12 aquatic mollusks, 5 of 

 them common to the Strawn-Batavia faunas. These are: Promenetus (Menetus) 

 exaouous , Valvata tricarinata, Eelisoma oampanutatum , Lymnaea obrussa, and 

 L. palustris. He also mentions Suooinea retusa, and several additional aquatics 

 by genus only. 



Reynolds (1959) described the molluscan fauna in a covered bog deposit 

 from a cornfield in Ohio; the recovered faunal assemblages were entirely aquatic. 

 He analyzes the various species as to percent of the total, but does not specify 

 whether the figures were arrived at from population numbers or by some other 

 means. La Rocque (1952) in an earlier study speaks of percent "by volume." 

 Reynolds uses Morrison's data from a study of molluscan faunas in Wisconsin 

 lakes (without reference), but strangely speaks of "fixed carbon dioxide ratio" 

 in parts per million. Reynolds accepts Morrison's data as if they established 

 the limits of tolerance of the various species concerned. Reynolds discusses 

 the absence of terrestrial elements in the faunas and concludes that terrestrial gas- 

 tropod shells could not drift as far as 150 feet, the approximate distance of the exca- 

 vation from the original soil. Reynolds reports the following species, which are com- 

 mon to the Strawn-Batavia faunal assemblages: Pisidium compression, P. nitidum, 

 Sphaerium sulcatum (= S. simile!) , Amnicola leightoni, Lymnaea obrussa, 

 Gyraulus altissimus , Eelisoma anceps (= antrosa) , Eelisoma trivolvis , Physa 

 gyrina, Promenetus exaouous , and Valvata trioarinata. In all, Reynolds reported 

 7 species of sphaeriids and 11 kinds of gastropods. 



Another significant study of ancient lake faunas was made by Zimmerman 

 (1960), who described 38 species of mollusks taken from the Newell Lake de- 

 posit in Logan County, Ohio. Among the 21 kinds of aquatic mollusca, 13 are 

 common to the Strawn-Batavia assemblages: Valvata lewisi, Amnicola leightoni, 

 Pseudosuccinea columella, Lymnaea obrussa, Eelisoma antrosa, E. campanulatum , 

 E. trivolvis, Planorbula armigera, Promenetus exaouous, Gyraulus altissimus, 

 Ferrissia parallela, and Physa gyrina. He also noted Succinea retusa. Zim- 

 merman also utilizes the data of Morrison on pH and "fixed carbon dioxide" as 

 if they established tolerance limits for various species. He compares his 

 faunas with Pleistocene molluscan faunas reported by Baker (1918). 



