Key:  Family Myidae

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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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

   Cryptomya californica
  Mya arenaria