The family Calpytraeidae is made up of three genera: Crepidula (slipper shells), Crucibulum (cup and saucer shells) and Calyptraea (hat shells). All of the species have limpet-shaped shells with an internal shelly septum. The genera are distinguished by shell morphology. In Crepidula the shell apex is directed posteriorly and the internal septum is a flat shelf that attaches along both sides of the shell. In Crucibulum the shell is conical with a central apex and a cup-shaped internal septum. In Calyptraea the shell is conical with a central apex but the internal shelly septum is a coiled ramp. Shells are often not diagnostic on the species level. Cryptic species often co-occur but they can usually be distinguished on the basis of developmental characters and body pigmentation. There are 6 species of Crepidula, one Crucibulum and one Calyptraea reported from Florida: Crepidula ustulatulina, C. depressa, C. atrasolea, C. aculeata, C. maculosa and C. fornicata, Crucibulum auriculum, and Calyptraea centralis.Crepidula atrasolea (formerly C. cf. plana) can be distinguished from other Crepidula species by the following suite of characters. The flat white shell ranges from recurved to somewhat convex depending on the habitat of the individual. Animals from exposed substrates are often oval and convex with a more robust shell. The shelf is flat in convex shells and convex in recurved shells,with a notch on the right side where it attaches to the shell and also a depression in the center of the shelf margin. Muscle scars are absent. Apex at the shell margin usually directly posterior, sometimes slightly recurved to the right. The shell is white, inside and out. The shell is 5 - 8 mm in length. There is no sculpture other than growth lines.There is diffuse to intense sooty black pigmentation over the foot, mantle, and neck. There are yellowish blotches on the mantle edge in some live animals. The sooty pigmentation is retained in recently preserved animals, but the yellow color is lost.Synonomy: Crepidula cf. plana Say, 1822