12 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487 



Valvata trioarinata possesses a shell shaped much like that of V. lewisi, 

 but the whorls are tricarinate, although the development of the carinae varies 

 greatly. This variation has led to the publication of many trinomial names, which are 

 essentially meaningless since many variations of the shell ornamentation occur 

 in the same population of the species. Furthermore, there is no correlation be- 

 tween variation in shell carinae and internal anatomy. If writers feel constrained 

 to refer to the variations in the development of the carinae, La Rocque has sug- 

 gested a means of doing this (1968, p. 368) which certainly is better than 

 trinomial designations. 



V. trioarinata forms a conspicuous element of each of the faunal 

 assemblages studied; it occurs in each of the 27 assemblages, and is in every 

 one of them the most numerously represented. It is present in the bottom samples 

 at each locality and was therefore one of the first species to colonize these bogs 

 as the original glacial lakes formed by meltwaters and by runoff from the 

 surrounding moraines. 



The species is widely distributed in North America east of the Rocky 

 Mountains, and was a conspicuous animal in Pleistocene fossil assemblages 

 on the Great Plains; it no longer lives there south of the Sand Hills area in 

 Nebraska . 



Family Amnicolidae 

 Genus Amnioola Gould and Haldeman 1841 



The genus Amnioola is composed of small branchiate gastropods having 

 tightly coiled spiral shells; the height of the shells is rarely more than 6 mm 

 and in most species is less. Although the genus is distributed primarily in 

 northeastern North America, living species extend as far southwest as eastern 

 Kansas. However, members of the genus thrive best in clear, cold waters. 

 Several genera are recognized in the family Amnicolidae (Berry, 1943), but only 

 one of them occurs in the bog faunas studied here. 



Amnioola gelida F. C. Baker 1921 



Amnioola lustrioa gelida F. C. Baker 1921, Nautilus, v. 35, p. 22. 

 Amnioola gelida?. C. Baker 1928, Freshwater Moll. Wisconsin, pt. 1, 

 p. 110, pi. 6, figs. 19-23. 



Although not as numerous as V. trioarinata in these collections, 

 Amnioola gelidaoccurs abundantly in every one of the faunal assemblages 

 studied. A. gelida is known only as a fossil that has been reported in Ohio 

 and in Illinois. The shells are high spiraled. Sexual dimorphism seems ap- 

 parent among them; the shells presumed to have contained male animals 

 possess a much broader ultimate whorl than do the presumptive females. This 

 is a common phenomenon among amnicolids, produced by the necessity of accom- 

 modating the large male intromittent organ (verge) . A. gelida was also an early 

 colonizer of the glacial lakes; the shells occur in the lowermost samples that 

 contain any fossils at all. 



Amnioola leightoni F. C. Baker 1920 



Amnioola winkleyi leightoni F. C. Baker 1920, Nautilus, v. 33, p. 125. 

 Amnioola leightoni F. C. Baker 1921, Nautilus, v. 35, p. 23. 

 Amnioola leightoni F. C. Baker 1928, Freshwater Moll. Wisconsin, 

 pt. 1, p. 120, pi. 6, figs. 34-39. 



