Taken primarily from Kozloff, 1987, 1996 p. 273 (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 | Only the right valve decidedly convex (the left valve is fairly flat); height commonly exceeding 10 cm | Patinopecten caurinus |
| 1b | Both valves decidedly convex; height less than 7 cm (except in the case of Hinnites giganteus (now Crassodoma gigantea), which reaches a height of more than 10 cm; this species, after reaching a height of about 1 cm, becomes permanently cemented to rock) | 2 |
| 2a | Interior of both balves with purple blotch; free-living until about 1 cm high, then becoming cemented to rock by the right valve; height often exceeding 10 cm | Crassodoma gigantea
(in key as Hinnites giganteus) |
| 2b | Interior of both valves without a purple blotch near the hinge; not permanently cemented to rock (but it may be temporarily attached by a byssus); height not more than 6 cm | 3 |
| 3a | With true external ribs (there may also be ruffles or spines, but if these are worn away, the ribs will still be evident); height commonly greater than 3 cm | 4 |
| 3b | Either without ribs or only with pseudoribs which consist of raised ruffles of the concentric ridges (if the ruffles, which may overlap one another, are scraped away, no rib remains) | 6 |
| 4a | Radial ribs of both valves with very prominent spines or ruffles, rough to the touch | Chlamys (Chlamys) hastata |
| 4b | Valves nearly or quite smooth to the touch (if the ribs have spines or knoblike excresences, these are so slight as to be invisible without magnification; the excresences, moreover, are generally restricted to the right valve) | 5 |
| 5a | Both valves with about 20-30 prominent ribs (ribs of the right valve with ruffles but those of the left valve smooth; common shallow subtidal species) | Chlamys (Chlamys) rubida |
| 5b | Both valves with about 10 especially prominent ribs, between which are less prominent ribs (rare subtidal species in our region) | Chlamys (Chlamys) behringiana |
| 6a | With raised internal ribs (height up to 2.5 cm) | Family Propeamussidae:
Parvamussium alaskensis |
| 6b | Without raised internal ribs | 7 |
| 7a | Either without pseudoribs or with pseudoribs that are about the same caliber as the concentric ridges (height up to 3 cm) | Delectopecten randolphi |
| 7b | With pseudoribs that are much more pronounced than the concentric ridges | Delectopecten vancouverensis |