1a

Pedicle (attachment stalk) absent; shell flattened (the thickness is less than half the width); lower valve firmly cemented to rock (thus this species could be confused with certain bivalve molluscs, such as Pododesmus cepio or small oysters) 

1b

Pedicle present; shell not especially flattened (the thickness is at least half the width); lower valve free of the substratum

2a

Shell dark gray or nearly black; posterior beak of the larger valve rather sharply pointed, its apex not obliterated by the opening through which the pedicle passes

2b

Shell yellowish, brownish or reddish; posterior beak of the larger valve not pointed, and its apex may be partly or wholly obliterated by the opening through which the pedicle passes 

3a

Shell usually wider than long; margins of valves at the anterior (broader) end (away from the hinge) with pronounced complementary undulations

3b

Shell usually longer than wide; margins of valves at the anterior (broader) end without pronounced complementary undulations (they may not, however, be perfectly straight, and they may have small teeth)

4

4a

Anterior margins of valves with regularly spaced small teeth; shell not translucent; valves with delicate, radiating striations, but usually without concentric growth lines; apex of the larger valve completely obliterated by the opening through which the pedicle passes

4b

Anterior margins of valves without regularly spaced teeth; shell somewhat translucent; valves with crowded concentric growth lines but without radiating striations; apex of the larger valve not completely obliterated by the opening through which the pedicle passes