Taken primarily from Kozloff, 1987, 1996 p. 283 (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 | Pallial sinus at least as deep as high (except in Sphenia ovoidea, in which it is not quite as deep as high); umbones at least slightly anterior to the middle; anterior and posterior halves of shell dissimilar | 2 |
| 1b | Pallial sinus much less deep than high; umbones almost exactly in the middle; anterior and posterior halves of shell similar (siphons opening into burrows of Upogebia and Neotrypaea, rather than at the surface) | Cryptomya californica |
| 2a | Chondrophore projecting only slightly, and elongated along the longitudinal axis of the shell; umbones decidedly anterior to the middle (anterior and posterior portions of valves very dissimilar, the posterior portion being slightly narrower and almost truncate; in burrows of other animals and in algal holdfasts) | Sphenia ovoidea |
| 2b | Chondrophore projecting nearly or quite to the extent of its width; umbones only slightly anterior to the middle | 3 |
| 3a | Posterior (siphonal) end of shell somewhat less smoothly rounded than the anterior end, but not truncate; length sometimes exceeding 10 cm; especially common in estuaries and other situations where the salinity is reduced by influx of fresh water | Mya arenaria |
| 3b | Posterior end of shell almost truncate; length rarely exceeding 7 cm; not usually found in situations where the salinity is reduced | 4 |
| 4a | Shell about 1.5 times as long as high; periostracum thick, brown, wrinkled, interior of valves yellowish, not pearly | Mya truncata |
| 4b | Shell almost twice as long as high; periostracum thin, yellowish, interior of valves pearly | Platyodon cancellatus |