Taken primarily from Kozloff, 1987, 1996 (Copyright 1996, University of Washington Press. Used in this web page by permission of University of Washington Press)
Jump to lower taxonomic level:
| 1a | Length about 1.25 times the height, radial ribs faint, though rather broad, and the surface of the shell nearly smooth (length up to 8 cm, but mostly less than 5 cm; subtidal) | Serripes groenlandicus |
| 1b | Length decidedly less than 1.25 times the height (sometimes just equal to or less than the height); radial ribs pronounced | 2 |
| 2a | Posterior quarter of valves with rather prominent concentric ridges and low radial ribs, so that the sculpture on this part of the shell is very different from that on the rest of the surface, where only the closely spaced radial ribs are distinct | Nemocardium (Keenaea) centifilosum |
| 2b | Valves without a substantial area in which the sculpture differs from that on the rest of the surface | 3 |
| 3a | Shell as high as or slightly higher than long; height up to about 8 cm; common intertidal and shall subtidal species | Clinocardium (Clinocardium) nuttallii |
| 3b | Length about equal or slightly greater than the height; height of most species not exceeding 4 cm (but in C. californiense it may nearly reach 7 cm); subtidal | 4 |
| 4a | Length about equal to the height (height up to about 3 cm; with over 50 radiating ribs, these rather delicate) | Clinocardium (Clinocardium) blandum |
| 4b | Length slightly greater than the height (ribs rather coarse) | 5 |
| 5a | With 32-38 radial ribs; height usually under 4 cm | Clinocardium (Clinocardium) ciliatum |
| 5b | With 40 or more radiating ribs; height may reach or exceed 4 cm | 6 |
| 6a | Height up to 7 cm; with about 40-45 ribs | Clinocardium (Clinocardium) californiense |
| 6b | Height up to slightly more than 4 cm; with about 45-50 ribs | Clinocardium (Clinocardium) fucanum |
Clinocardium (Clinocardium) nuttallii