MAZATLAN UNIVALVES 277 
it is not impossible that further researches may prove their 
identity. 
The vertex of C. incurva is discoidal and smooth, as in C. 
onyx, rather smaller, and more hidden by the subsequent 
growth of the shell. The earlier portion is very convex, smooth, 
nearly black. It develops ridges rather suddenly, strong 
enough to serrate the margin, which is then beautifully varie- 
gated with horn colour and tortoiseshell, and frequently rather 
spreading. As the shell advances, the ribs often pass away, 
leaving traces however by which the species may generally be 
recognized. The deck occupies a larger or smaller proportion 
in different specimens, and is more or less sunken, rarely 
developing a slight hole under the vertex, approaching C. 
aduneca. The spreading specimens generally assume a hehter 
colour, like C. onyx, which in mode of growth and m the 
character of the deck they closely resemble. A large propor- 
tion of the shells are truncated in front, as though from 
living against arising surface. The youngest specimen is ‘O07 
in length; alarge one, approaching C. onyx, and living hke 
an operculum in the mouth of Uvanilla unguis, measures 
long: °9; lat. °6,* ali. “25 
A convex, spreading sp. sat a ODE tO Pant mean 
A narrow sp. so, URGES | ony OS ee kate ese 
Hab.—St. Elena and Xipixapi; on dead shells in sandy mud 
6—10 fm. ; Cuming.—Payta, Fontaine —Panama ; on living 
Gasteropods at and near low water mark, and on each other, 
common; C. B. Adams.—San Blas, Capt. Wendé, (Menke.)— 
Mazatlan ; on shells and on each other, not uncommon ; 
“pool Col. 
Characters approaching C. onyx. Tablet 1362 contains the 
specimen in U. unguis.—1363, 1 sp. nearly smooth, singularly 
indented. — 1364, 3 sp. scarcely striated, flat. — 1365, 3 do. 
convex, laterally compressed.—1366, 3 sp. slightly striated, 
flattened, light coloured.—1367, 3 do. very dark.—1368, 3 do. 
more stria ated, light colour. 
Normal state. Tablet 1369 contains 3 sp. very young.— 
1370, 6 sp. different ages, deck margin deeply angulated.— 
1371, 4 sp. slightly do.—1372, 4 sp. slightly sinuated.—1373, 
A sp. flattened, broad.—1374, 7 do. lighter colour, deck margin 
nearly straight, direction slanting.—1375, 6 sp. deep, compress- 
ed growth.—1376, 4 sp. twisted, vertex at the left.—1377, 3 do. 
vertex at the right.—1378, Columbella fusca, with Crepidula 
incurva, shewing En Se —1379, Pisania hemastoma, with 
July 1856, an 
