52 



FRANK COLLINS BAKER 



TABLE I 

 Distribution of Species according to Geologic Age 



Species 



Sangamon 



Peorian 



Wabash 



Bivalves 



Musculium rhomboideum 



Pisidium costatum 



Pisidium species 



Fresh-water gastropods 



Pomatiopsis lapidaria 



Valvata sincera 



Aplexa hypnorum 



Pkysa gyrina 



Planorbis altissimus 



Planorbis urbanensis 



Galba palustris 



Galba obrussa 



Galba dalli 



Galba parva 



Land gastropods 



Carychium exile 



Helicina occulta 



Vallonia gracilicosta 



Succinea ovalis 



Succinea avara 



Succinea vermeta 



Vertigo modesta 



Pupilla muscorum 



Gastrocopta armifera 



Gastrocopta tappaniana 



Strobilops virgo 



Sphyradium edentuliim alticola. 



Helicodiscus parallelus 



Pyramidula shimekii 



Oreohelix iowensis 



Euconulus fulvus 



Vitrea rhoadsi 



X 



X 



X 



X 



X 



X 



FRESH-WATER BIVALVES (PELECYPODs) 



Musculum rhomboideum (Say). A single valve of this fragile 

 species was found in a deposit of blue-gray silt of post-Illinoian and 

 pre-early Wisconsin age (Sangamon?) near Irene, Boone County. 

 It is apparently rare in Pleistocene deposits. 



Pisidium costatum Sterki. This small bivalve was selected 

 from three deposits at two localities; in yellow sand and blue-gray 

 silt near Irene, Boone County (Sangamon age), and in the blue 

 silty clay in Whiteside County (Wabash age). It is abundant 

 near Irene, but rare in Whiteside County. Costatum is known only 

 from Pleistocene deposits, not yet having been detected among 



