352 



THE CHICAGO ACADEMY OF SCIENCES. 



so appressed at its junction with the parietal wall as to form a pseudo- 

 plait; the inner edge of the outer lip frequently forms a rib-like varix 

 in very old specimens. 

 Length. 



3.25 



4.00 



3.00 



4.2.". 



4.50 



3.75 



4.00 



Types: Chicago Academy of Sciences. Xo. 23125; cotypes, 

 Academy of Xatural Sciences. Philadelphia ; Smithsonian Institution, 

 Washington. 



Type Locality : Marsh, west side Lake James, Steuben County, 

 Indiana. 



Animal. Taw. Radula and Genitalia : Xot examined. 



Breadth. 



Aperture. 



Brea 



dth. 





2.00 



1.50 



1.00 



mill. 



(Lake James) 



2.00 



2.00 



1.00 



" 



" 



2.00 



1.25 



.HO 





" 



2.00 



1.75 



1.00 



" 



it a 



2.50 



2.00 



1.10 





(Scalariform) 



2.75 



2.00 



1.25 





Rockford 



2.10 



2.00 



1.10 





" 



Fig. 24. 



Range (Figure 24) : Ohio to northern Michigan and Montana, 

 south to Kansas and Arizona. 



In general, dalli occupies a large part of the Transition and Lpper 

 Austral life zones ; it reaches the Canadian zone in Michigan. It is 

 typically an inhabitant of the Upper Mississippian region, reaching 

 here its greatest degree of perfection. It has migrated south and west 

 into the Coloradoan region. So little is known concerning the distri- 



