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 G. 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 G. Sinensis; an extraordinarily large one 
measures long. 1°26, lat. 1°23, alt. *56. 
Hab.—Isl. Muerte; on dead shells, sandy mud, 11 fm.; Cwm- 
éng.—(G. lichen.) Idem, Cuming. — (G. regularis) Panama ; 
extremely rare; C. B. Adams.—Acapuleo, Col. Jewett.— 
Sta. Barbara, Col. Jewett——Mazatlan ; not uncommon, ad- 
hering to shelis, and to each other; L’pool Col.—Payta to 
Guayaquil, D’ Orbigny. 
Tablet 1318 contains 6 pale specimens.—1319, 6 do. dark 
colour, (one in situ.)—1320, the largest specimen. 
Genus CREPIDULA, Lam. 
Crepidula, Zam. 1799.— Phil. Handb. Conch. p. 188. 
Sandalium, Schwm. 1817. 
Crypta (Humph. 1797) Gray, 1847: H. & A. Ad. Gen. vol. 1. 
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 Calyptrea levigata, Lam.: v. 
Deless. Rec. Cog. Lam, pl. 25, f. 3.a,b,c. The C. extinctorium, Lam. (quoted by 
Sow. for a totally different shell) appears from the same work, Joc. cit. f. 2, a, b, 
ec. to be a somewhat similar, blackish Galerus, in which however the outside and 
the inside representations of the spiral element do not agree, 
