286 GASTROPODA. [Pectinibranchia. 



Genus 2. CREPIDULA, Lamarck, 1799. 



Crepidula, Lam., Mem. Soc. Hist. Nat. Paris, 1799, 78. Type : Patella for ni- 

 cata,it. Proscenula, Perry, 1811. C'rypta, Humphrey, 1797 (name only). 



Animal with head depressed, laterally dilated, muzzle short, 

 bilobed ; tentacles short, subulate ; foot short, subtruncate in front, 

 rounded behind. 



Shell oval, patelliform, with a posterior generally lateral spiral 

 apex ; interior with a basal plate covering the posterior half of the 

 aperture. 



Adhering to shells or stones, and modifying the form and surface 

 in accordance with the inequalities of their place of attachment. 



The distribution is world- wide ; the individual species have a 

 wide range, which, added to their great variability, as in attached 

 shells generally, has caused an enormous specific synonymy. 



Fossil. Cretaceous and Tertiary. In the Miocene the genus 

 exhibits luxuriance both in size and variation. 



Vernacular XHHH-. Slipper-limpet. 



KEY TO SPECIKS. 



A. Upper surface nearly smooth, high convex or flattened, sometimes 



concave . . . . . . . . . . . . crepidula. 



B. Upper surface with radiating nodulous or spino.se ridges . . costata. 



1. Crepidula crepidula, Linne, 1764. Plate 44, figs. 5. 5a. 



Patdla crepidula, L., Mus. Lud. Ulricse, 1764, 689; Favanne, "Conchylio- 

 logic," pi. 4, f. D. Crepidula unguiformis, Lamarck, A.s.V., vii, 1822, 642. 

 Calyptrcp.a (Crepidula) monoxyla, Lesson, Voy. Coq., ii, 1830, 391. Crepi- 

 dula monoxyla, Lesson, Man. Conch. (1), viii, 128, pi. 37, f. 35, 36 ; 130, 

 pi. 39, f. 66-68. C. contorta, Q, & G.. Voy. Astrol., iii, 1835, 418, pi. 72, 

 f. 15, 16. Crypta profunda, Hutton, C. Tert. M., 1873, 14. 



SkcU oval or oblong, high convex, flattened or concave above, 

 often twisted, nearly smooth, but frequently somewhat lamellar. 

 Sculpture consisting of close concentric growth-lines, slightly lamellar 

 anteriorly ; crossed by fine and close microscopic radiate striae, dis- 

 tinctly visible on the neanic part of the shell, but very often obsolete 

 on the remainder of the upper surface. Colour white or yellowish- 

 white. Epidermis very thin, horny, easily worn off. Protoconch of 

 one-half whorl onlv, flatlv convex and smooth ; in voung shells it is 



t/ ^' ./ O 



at the posterior fourth, but in adult shells it is mostly terminal. Shells 

 living on stones or other shells are flatly or very highly convex, those, 

 however, living in the interior of dead gastropod shells are quite flat 

 or concave above. Interior porcellanous, highly polished ; the pos- 

 teriorly situated basal plate convex, with a sharp slightly concave 

 margin, the length of the plate being about one-fourth of the length 

 of the shell. 



Length of adult shell, 20-25 mm. ; the breadth and height are 

 extremelv variable. 



