PLEISTOCENE MOLLUSC A FROM ILLINOIS 53 



living Pisidia. It is widely distributed and has been recorded 

 from Aroostook County, Maine (Baker, 19206, p. 155), Michigan 

 (Sterki, 1916, p. 466), and Urbana, Illinois (Baker, 1918, p. 663). 

 These records are all Wabash (post- Wisconsin) . The present 

 records carry the species back into interglacial time. 



Pisidium, species indeterminate. A few odd valves of a small 

 Pisidium occurred in peat material from a flood-plain pond in 

 Carroll County, of Wabash age (post- Wisconsin) . 



FRESH-WATER SNAILS (gASTROPODS) 



Pomatiopsis lapidaria (Say). A mile north of Ridott, Stephen- 

 son County, in loesslike silt, this species is abundant in company 

 with Galha ohrussa and Helicodiscus parallelus. The species does 

 not differ in any way from living forms. 



Valvata sincera (Say). Abundant in blue silts in Whiteside 

 County, apparently of Wabash age. These individuals are quite 

 typical in everything except size, the largest specimens being 

 almost I mm. less in diameter than the largest living sincera from 

 Lake Winnipeg. The two measurements are: 



Altitude D iameter 

 mm. mm. 



Recent species from Lake Winnipeg 4.5 5.5 



Fossil species from Carroll County 3.5 4.5 



Sincera from Urbana deposits (Baker, 1918, p. 663) are like 

 the Carroll County forms in size. If this disparity in size should 

 prove universal, it might be of advantage to differentiate the latter 

 as a fossil race. Not enough material of either the Recent or the 

 fossil sincera is at hand to settle this question definitely. Like the 

 Recent forms, the fossil specimens vary somewhat in the height of 

 the spire and in the sculpture of the whorls, there being a tendency 

 in a few individuals to form thin, sharp ribs, as in the variety 

 nylanderi. This feature is more marked in the Urbana sincera 

 than in the specimens from Carroll County. 



PULMONATE GASTROPODS 



Physa gyrina Say. Rare in the Carroll County deposit, the 

 specimen found being also immature. 



