Taken primarily from Kozloff, 1987, 1996 p. 216 (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 | Diameter about two-thirds of the height; height of aperture about half or slightly less than half the total height of the shell | 2 |
| 1b | Diameter more than two thirds of the height; height of aperture about two-thirds the total height of the shell | 3 |
| 2a | Height of aperture usually slightly less than half the height of the shell (common shallow subtidal species, and occasionally found intertidally) | Trichotropis cancellata |
| 2b | Height of aperture about half, or slightly more than half, the height of the shell (rare subtidal species) | Trichotropis borealis |
| 3a | With 2 pronounced spiral ridges on the body whorl, these not separated by smaller ridges; lowermost part of aperture rather sharply angled; height up to 5 cm (rare subtidal species; not likely to be found south of British Columbia | Trichotropis bicarinata |
| 3b | With 2 pronounced ridges on the body whorl, these separated by several smaller ridges; lowermost part of aperture not sharply angled (the outline of the aperture is nearly circular); height to 2.5 cm (rare subtidal species) | Trichotropis insignis |