Taken primarily from Kozloff, 1987, 1996
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| 1a | Shell conical, almost circular in outline, with a spiral septum (shelf) the free edge of which is twisted | Calyptraeea fastigiata
Gould, 1846 |
| 1b | Shell usually cap-shaped (sometimes nearly conical), generally not completely circular in outline and often slightly irregular; with a simple septum | 2 |
| 2a | Septum attached more broadly on one side (the left side, as the aperture faces the observer and the shelf is uppermost), so that part of it projects like a tongue | Crepidula dorsata
(Broderip, 1834) |
| 2b | Septum attached to about the same extent on both sides, its free margin therefore nearly straight | 3 |
| 3a | Shell definitely arched (often higher than wide), the apex prominent and beaklike; periostracum dark brown; on shells of living Tegula, Calliostoma, and other gastropods | Crepidula adunca
Sowerby, 1825 |
| 3b | Shell low (never as high as wide), the apex not prominent and often not even distinct; exterior whitish or with a light periostracum, but not dark brown; frequently on empty shells of bivalves or gastropods, including those occupied by hermit crabs, but not attached to the shells of living Tegula, Calliostoma, or other gastropods | 4 |
| 4a | Apex of shell directed laterally and united with the margin (introduced with oysters from the Atlantic coast, and found in only a few areas) | Crepidula fornicata
Linnaeus, 1758 |
| 4b | Apex of shell directed posteriorly, often overhanging the posterior margin of the shell | 5 |
| 5a | Shell whitish, generally almost flat, with a shaggy, yellow-brown periostracum | Crepidula nummaria
Gould, 1846 |
| 5b | Shell whitish, almost flat, concave, or misshapen (depending on habitat), with a thin, brown periostracum or without any obvious periostracum (often in burrows of pholadid bivalves, in empty shells, and inside shells occupied by hermit crabs, fitting the contour of the concave surface) | Crepidula perforans
(Valenciennes, 1846) |