﻿MARGARITANA 513 



KkY to SPECIKS OF MaRGARITANA. 



Posterior end of the shell connigated. M. hembeli. 



Shell not corrugated. 



Pseudocardinals strong. 



Laterals generally wanting. M. margaritifera. 



Laterals present. 



Shell large, solid. M. crassa. 



Shell rather small, thin behind. M. laosensis. 



Pseudocardinals feeble. 



Shell long, not compressed, arcuate. M. monodonta. 



Shell rather short, compressed, M. decumbens. 



Group of Margaritana margaritifera. 



Shell having irregular, concentric growth lines, but without 

 plicate or corrugated sculpture. 



Margaritana margaritifera (Linnaeus). 



Shell large, much elongated, usually arcuate, rounded be- 

 fore and somewhat so behind, rather solid, inequilateral, not 

 inflated ; beaks low, not inflated, their sculpture consisting of 

 longitudinal ridges, which are sometimes a little broken ; pos- 

 terior ridge moderate or low, often having a wide, radial de- 

 pression above it, and ending below the median line, some- 

 times in a point, in old shells in a rounded termination ; surface 

 with rude growth lines ; epidermis thick, blackish or brown- 

 ish, often subshining ; left valve with two stumpy pseudocar- 

 dinals ; right valve with one pseuidocardinal ; laterals generally 

 wanting; beak cavities not deep; muscle scars impressed, the 

 anterior rough, the posterior elliptical ; nacre lurid, bluish- 

 white, whitish, purplish or purple, generally showing pits 

 where the mantle is attached, often having dark blotches. 



Length 152, height 65, diam. 40 mm. 



Length 122, height 60, diam. 38 mm. 



Length no, height 54, diam. 35 mm. 



.\11 Europe except the southernmost portion ; northern Asia : 

 Japan ; northern North America ; Iceland. Its southern limit 



