EXPLANATION OF PLATES. 

 Plate I. 



1— Unio f;il)nlis; 2 — I'nio ghms: 3— L'lilo siil)rostr;itiis; 4— Unio nil»i- 



giiiosiis; 5— Mar.iiiiritaiia luarginatii: G— L^nio luteolns; 7— Anotlouta 

 grandis: S — AnotTonta edeiitnlu.s. 



Plate II. 



1, 2, 3, and 4 are pairs of U. liiteolus, which exhibit gradations of form, 

 -color and size from the light straw-colored forms to the almost black 

 variety. 



a, 1), c, d, e, f. g. and h exhibit the gradations of color and niiuiungs 

 ;found, from white to darli varieties, witliout regard to sex. 



