Taken primarily from Kozloff, 1987, 1996 p. 278 (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 | Width of gape at posterior end more than 1/4 the width of the shell; length sometimes attaining 20 cm; burrowing in mud and clay, intertidal and subtidal | 2 |
| 1b | Width of gape at posterior end less than 1/4 the width of the shell; length rarely exceeding 10 cm; mostly in sand, and primarily subtidal | Spisula (Symmorphomactra) falcata |
| 2a | Shell generally about 1.5x as long as high; umbones near the end of the anterior third; common and widely distributed in our region | Tresus capax |
| 2b | Shell more than 1.5x as long as high; umbones near the end of the anterior quarter; uncommon in our region | Tresus nuttallii |