MAZATLAN UNIVALVES 267 



about '02 across, generally rather separated from the shell, 

 and of an elegant discoidal shape like Planorbis, displaying the 

 whirls and the sunken apex. The whirls increase rapidly and 

 regularly, sometimes developing concentric wrinkles (as in 

 Crep. nivea,) just beyond the nuclear portion. The internal 

 lamina makes a more complete revolution than in G. conica ; 

 the central margin is more produced, and the reflection is only 

 over half the breadth. The Mazatlan shells differ from G. 

 Sinensis and its W. Indian congener, in the reflection of the 

 lip, which is nowhere closed, in the character of the nucleus, 

 and in the non-spinous surface.* The colour is often of a rich 

 brown within and near the vertex ; otherwise of a dingy white. 

 Epidermis very thin. The cone is more or less depressed. 

 The form Gr. lichen is probably only a flattened variety of this 

 species ; but may be distinct. The C. mamillaris of D'Orb. is 

 the C. unguis of Brod. and appears a distinct species, of which 

 the C. sordida of Brod. is probably a variety : the lamina being 

 so much reflexed as to pass into Crucibulum, the young of which 

 however it does not at all resemble. The ordinary size of G. 

 mamillaris is as in Gr. Sinensis ; an extraordinarily large one 

 measures long. 1*26, lat. 1'23, alt. '56. 



Hob. Isl. Muerte ; on dead shells, sandy mud, 11 fm. ; Cum- 

 ing. (Gr. lichen.) Idem, Cuming. (Gr. regularis) Panama ; 

 extremely rare; C. B. Adams. Acapulco, Col. Jeivett. 

 Sta. Barbara, Col. Jewett. Mazatlan ; not uncommon, ad- 

 hering to shells, and to each other ; Lpool Col. Payta to 

 Guayaquil, IfOrbigny. 



Tablet 1318 contains 6 pale specimens. 1319, 6 do. dark 

 colour, (one in situ.) 1320, the largest specimen. 



GENUS CKEPIDTJLA, Lam. 



Crepidula, Lam. 1799. Phil. Handb. Conch, p. 188. 



Sandalium, ScJium. 1817. 



Crypta (Humph. 1797) Gray, 1847 : H. <$f A. Ad. Gen, vol. i. 



p. 368. 



The species of this genus run into each other in the most 

 marvelous manner. The characters even of the young shell 



* The W. Indian analogue of this species is Calyptrsea laevigata, Lam. : T. 

 Deless. Rec. Coq. Lam. pi. 25, f. 3 a, b, c. The C. extinctorium, Lam. (quoted by 

 Sow. for a totally different shell) appears from the same work, loc. cit. f. 2, a, 5, 

 c. to be a somewhat similar, blackish Galerus, in which however the outside and 

 the inside representations of the spiral element do not agree. 



