Key:  Family Calyptraeidae (slipper shells)

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)