Taken primarily from Kozloff, 1987, 1996 p. 280 (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 | With distinct radial ribs (these may, however, be delicate or faint) | 2 |
| 1b | Without radial ribs | 4 |
| 2a | Radial ribs typically more prominent than the concentric ridges (but this is not always the case); hinge ligament about 1/3 the length of the shell; length not more than 6 cm | 3 |
| 2b | Radial ribs less prominent than the concentric ridges; hinge ligament nearly half the length of the shell; length sometimes attaining 8 cm | Protothaca (Callithaca) tenerrima |
| 3a | With a row of small teeth on the inside of the valves close to their ventral margins; interior of valves whitish, without a purple stain near the posterior end | Leukoma staminea |
| 3b | Without a row of teeth on the inside of the valves close to their ventral margins; interior of valves (often) yellowish, with a purple stain near the posterior end. [In key as Tapes (Ruditapes) philippinarum] | Venerupis philippinarum |
| 4a | Length less than 8 mm; height usually more than 3/4 the length; angle formed by the dorsal margin at the umbones not more than 110 degrees; 3 cardinal teeth on the hinge plate of both valves; embryos brooded internally | 5 |
| 4b | Length generally greater than 2 cm; height not often more than 3/4 the length; angle formed by the dorsal margin at the umbones more than 110 degrees | 8 |
| 5a | ||
| 5b | ||
| 6a | ||
| 6b | ||
| 7a | ||
| 7b | ||
| 8a | Concentric ridges conspicuously raised; posteriormostcardinal tooth on right valve not bifid | 9 |
| 8b | Concentric lines of growth not conspicuously raised, the exterior of the shell thus nearly smooth; posteriormostcardinal tooth on right valve bifid (length up to about 6 cm) | Compsomyax subdiaphana |
| 9a | Umbones at the end of the second 1/5 of the shell from the anterior end; periostracum yellowish, polished; length up to 2.5 cm; subtidal | Liocyma fluctuosa |
| 9b | Umbones at the end of the first 1/5 of the shell from the anterior end; periostracum not yellowish and not polished; length generally exceeding 3 cm | 10 |
| 10a | Shell closing tightly at the posterior end; concentric ridges bent in the direction of the umbones; length up to 10 cm; subtidal | Humilaria kennerlyi |
| 10b | Shell gaping slightly at the posterior end; concentric ridges, even if sharp, not bent in the direction of the umbones; length often exceeding 10 cm; common intertidally | Saxidomus gigantea
(S. giganteus in key) |
Humilaria kennerlyi
Leukoma staminea
Saxidomus gigantea
Venerupis philippinarum
Introduced, not in key:
Mercenaria mercenaria