12 University of Michigan 



PLATE III 



Figure 12. C. exiguum. Independence Lake, Washtenaw County, 

 Michigan. 



Figure 13. C. exiguum. Ann Arbor, Michigan. Very sinuate 

 lamellae. 



Figure 14. C. exiguum. Ann Arbor, Michigan. 



Figures 14 and 15. C. exiguum. Edgeworth, Pennsylvania. G. H. 

 Clapp. Very short, stumpy shell. 



Figures 16 and 17. C. occidentalis Pils. Seattle, Washington. G. 

 H. Clapp. 



