586 MOLLUSCA 



FAMILY 10. CYKENIDAE. 



Shell small, oval or more or less triangular, thin, greenish or yellow- 

 ish in color; 1 to 3 cardinals and both anterior and posterior laterals in 

 each valve ; small pallial sinus or none ; foot elongate, flattened ; ligament 

 external; eggs hatched in the inside pair of gills; mantle open below 

 with more or less complete siphons : several hundred species, all in fresh 

 or brackish water, living usually in small streams, ponds, and lakes, 

 either in or on sand or mud or on submerged plants; the young have a 

 byssus by which they often suspend themselves. 



Key to the genera of Cyrenidae here described: 



G! Umbo median or towards the anterior end ................... 1. SPH^ERIUM 



o a Umbo towards posterior end ................................ 2. PISIDIUM 



1. SPHJERIUX Scopoli (Cyclas Bruguiere). Shell thin, ovoid, green- 

 ish; cardinals small or rudimentary, 1 arched cardinal being in the right 

 and 2 in the left valve; laterals compressed; umbo usually a little in 

 advance of the middle; siphons elongate, of unequal size, the branchial 

 being the largest, and neither being closed except at the base : 75 species ; 

 cosmopolitan; about 20 in this country. 



Key to the species of Sphcerium here described: 



! Umbo in front of the center; sliell solid. 

 &! Lines of growth coarse .................................. S. STAMINEUM 



6 2 Surface almost smooth .................................. S. STRIATINUM 



a a Umbo median. 

 &! Shell fragile, light-colored ................. . ............. S. OCCIDENTALE 



6, Shell solid, dark-colored .................................. S. SULCATUM 



S. stamineum (Conrad). Shell large, inflated, oval, 14 mm. long, 

 11 mm. high, and 9 mm. wide; surface shining, yellowish or brownish in 

 color, with coarse lines of growth; umbo made prominent by a number 

 of coarse, rounded ridges: eastern and central states; common. 



S. striatinum (Lamarck). Shell elongated, yellowish in color, 10.5 

 mm. long, 7.5 mm. high, 6 mm. wide, with a smooth shining surface and 

 delicate lines of growth: entire country; common. 



S. occidentale Prime (Fig. 923). Shell round, with the umbo median 

 small, fragile, inflated, light yellowish in color, 7.5 mm. 

 long, 7.7 mm. high, 4.5 mm. wide; surface shining, with 

 very fine lines of growth: entire country; very common. 

 S. sulcatum* (Lamarck). Shell large, oval, inflated, 



(Baker?! 6 rather solid, with coarse lines of growth, 18 mm. long, 13 

 mm. high, and 10 mm. in width; surface smooth and dark 

 brown in color ; lateral teeth in a straight line with the cardinals instead of 

 at right angles to them : entire country ; not so common as the others. 



* See "Anatomy of Sphaerium sulcatum Lam.," by G. A. Drew, Proc. Iowa Acad. 

 Sci., Vol. 3, p. 173, 1895. 



