Description. Shell elongate, about twice as long as broad, moderately inflated, white; apex at posterior margin, slightly deflected to right; septum extending half length of shell, undulating on left side; periostracum light brown, thin, closely adherent. Length 15-23 mm.
Type Locality and Type Specimens. San Pedro, Los Angeles County, California. Lectotype (here designated): USNM 183455. Material Examined. Santa Maria Basin, Phase I, sta. 64,59 m (2). Other material examined: 12 lots in LACM collection.
Distribution. Point Conception (34°N), to Laguna Beach, Orange County, California (33°N).
Habitat. Living on the sides of the burrowing brachiopod Glottidia alba Hinds, 20-70 m. It assumes a convex form to match that of the outer surface of the Glottidia, but does not grow larger than its host.
Remarks. This species has usually been dismissed as a form or variety of some other species since it was first described by Dall. Hoagland (1977:393) placed it under Crepidula nummaria Gould, 1846 [for which see Abbott, 1974:142, fig. 1571]. I tentatively recognize it here to call attention to the fact that this question needs to be investigated by means of life history studies and biochemical genetics. As Hoagland notes (1977:393), all of the white-shelled, planar species of Crepidula from the eastern Pacific are in need of attention. Typical specimens of Crepidula nummaria have a thick periostracum and are more oval at all growth stages. The fact that very small specimens of C. glottidiarum are relatively elongate suggests that the species may be more than a situs form of C. nummaria.