446 FRANK COLLINS BAKER 



in Lake Michigan, off Racine, Wisconsin, at a depth of thirty- 

 fathoms. Additional records from both recent and fossil faunal 

 areas are needed to understand the distribution of this tiny species. 

 Geologists or others who discover such deposits should carefully 

 collect the material, noting rather minutely the stratigraphy, and 

 send the material, unsorted, to some competent malacologist for 

 study. Such deposits occur plentifully in Iowa, Wisconsin, Illinois, 

 Michigan, Indiana, Ohio, and Maine, and also in parts of other 

 states which were overridden by the great ice sheets. 



The material described in this paper forms a part of the Pleis- 

 tocene collection of the Museum of Natural History of the Uni- 

 versity of Illinois. 



ANNOTATED LIST OF MOLLUSCA FROM THE POSTGLACIAL DEPOSITS 

 NEAR RUSH LAKE, LOGAN COUNTY, OHIO 



Unionidae 

 Anodonta species. Fragments of a naiad, apparently a thin- 

 shelled Anodonta, occur with the material. Evidently rare, as but 

 few fragments were found. 



Sphaeriidae 



Sphaerium sulcatum (Lamarck). This large Sphaerium is 

 abundant in the material from the Ohio deposit and is the only 

 member of the genus found. These shells vary in form more than 

 do most individuals of the recent fauna. . 



Musculium rosaceum (Prime). A dozen odd valves of a Mus- 

 culium are referred to this species by Dr. Sterki, who says: ^''Mus- 

 culium, different forms, but apparently of rosaceum, deformed." 



Pisidium compressum Prime. A common, almost abundant 

 species in this marl bed, but none typical. Sterki says: "near 

 variety laevigatum. " 



Pisidium variabile Prime. About as common as P. compressum. 

 Sterki states that this species is difficult to separate from compres- 

 sum, especially among fossil individuals. This fact would indicate 

 a common origin for both species, and the study of the Pleistocene 

 material is, therefore, very important from the standpoint of 

 geological evolution. 



