Taken primarily from Kozloff, 1987, 1996 (Copyright 1987, 1996, University of Washington Press. Used in this web page by permission of University of Washington Press)
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| Lower taxonomic level | Main Page | Alphabetic Index | Systematic Index | Glossary |
| 1a | Umbones close to the posterior end of the shell and inclined posteriorly (unusual for a bivalve); length not more than 5 mm (free-living in sediment or associated with burrowing crustaceans and polychaetes) | Mysella (Rochefortia) tumida |
| 1b | Umbones near the middle of the shell and inclined slightly anteriorly; length up to nearly 2 cm | 2 |
| 2a | Dorsal margin (near the hinge) of right valve as smooth as that of the left valve; thickness more than one-third of the length; ventral margin (opposite the hinge) generally either straight or indented (attached by a byssus to the ventral side of the abdomen of the decapod crustacean Upogebia pugettensis and to the ventral surface of polychaetes of the genus Aphrodita) | Pseudopythina rugifera |
| 2b | Dorsal margin of right valve serrated, especially posterior to the umbo; thickness about 1//3 of the length; ventral margin convex to nearly straight, but not indented | Pseudopythina compressa |