114 



GEOLOGY OF LA SALLE COUNTY. 



rounded in adult specimens, convex sub-equipartite, 

 beaks slig'htly elevated, surface with conspicuous con- 

 centric wrinkles, epidermis brown or chestnut color. 

 About seven-tenths of an inch long-, heig'ht one-half 

 inch, breadth two-fifths of an inch. 



Abundant in Illinois river. Shells bluish within. 

 The young' shells are of a lig'hter color, thinner, and 

 show the wrinkles at all ag-es; lines of growth promi- 

 nent. iVnimal with simple mantle; foot tongue-shaped. 



S. sidcatiun, Lam., is the same shell. 



Sphasrium partumeium, Say. — Shell rounded, 

 oval, sub-equipartite, lowest anteriorly, somewhat 

 angular behind, thin and fragile, valves very convex, 

 minutely wrinkled by lines of growth, and obsoletely 

 radiated, light horn color, beaks elevated. Length 

 nine-twentieths of an inch. Abundant. 



Sphaerium occidentale. Prime. — Shell small, oval, 

 thin, fragile, striae fine, beaks small and rounded, 

 color lig-ht yellow, beaks not prominent. Length one- 

 third of an inch- Not so abundant as the preceding- 

 species. 



GENUS PISIDIUM, PF'r. 



Pisidium abditum, Prime.— Shell small, rounded, 

 oval, beak raised slig-htly, surface smooth, stri^ fine, 

 color chestnut, beaks near the posterior side. Length 

 three-twentieths of an inch, breadth one-tenth of an 

 inch. 



This widely distributed species is abundant, but 

 from its small size seldom noticed. Pound in mud in 

 rivers. 



Pisidium compressum, Prime— Shell solid, trigo- 

 nal, very oblique, drawn up near the beaks which are 

 placed posteriorly, striae distinct, epidermis chestnut. 

 Size about the same as P. abditum. Abundant in 

 swales and Illinois river. 



