XV111 ANALYTICAL KEY. 



Two pairs of adductor 7tiuscle$. 

 Bearing a pearly shell, mantle freely open except behind. 



Shell equivalve, very inequilateral, auriculated behind, thin, toothless ANODON. 



Shell ovate or oblong, compressed or ventricose, green or fulvous-olive ... — cygnea. 



Shell equivalve, very inequilateral, slopingly produced behind, toothed ... UNIO. 



Shell ovately wedge-shaped, olive-green, rayed, umboes wrinkled — tumidus. 



Shell elongately oblong, fulvous olive, umboes faintly wrinkled — pictorum. 



Shell oblong kidney-shaped, black, umboes more or less eroded — margaritifer. 



Bearing a thin homy shell, mantle closed at each end. 



Branchial and excretory siphons prolonged into tubes uuited in one PISIDIUM. 



Shell large, triangular, posteriorly slopingly produced — amnieum. 



Shell very small, suborbicularly ovate, umboes obtuse, prominent — obtusale. 



Shell very small, obliquely oval, umboes moderately tumid — pusillum. 



Shell very small, suborbicular, glossy, umboes obtuse, grooved — nitidim. 



Shell small, triangularly orbicular, posteriorly obliquely produced — Casertanum. 



Shell minute, obliquely orbicular, anteriorly obliquely produced — pulchellwm. 



Shell minute, obliquely sub triangular, umboes calyculate — Henslowianum. 



Branchial and excretory siphons prolonged into tubes, partially united CTCLAS. 



Shell large, oval-globose, striated, slightly lunuled, ligament apparent — rivicola. 



Shell small, suborbicular, ligament scarcely apparent — cornea. 



Shell moderate, oblong-oval, posteriorly subtriangularly sloped — Pisidioides. 



Shell moderate, oblong-oval, pale, umboes nearly central — ovalis. 



Shell small, squarely orbicular, rhombic, thin, umboes calyculate — lacustris. 



