MAZATLAN UNIVALVES 277 



it is not impossible that further researches may prove their 

 identity. 



The vertex of C. incur va 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. 

 adunca. The spreading specimens generally assume a lighter 

 colour, like C. onyx, which in mode of growth and in the 

 character of the deck they closely resemble. A large propor- 

 tion of the shells are truncated in front, as though from 

 living against a rising surface. The youngest specimen is '07 

 in length ; a large one, approaching C. onyx, and living like 

 an operculum in the mouth of Uvanilla unguis, measures 



long. '9, lat. '6, alt. '25. 



A convex, spreading sp. '92, *67, *4. 



A narrow sp. '82, '46, '33. 



Hob. 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. 13. Adams. : San Bias, Capt. Wendt, (Menke.) 



Mazatlan ; on shells and on each other, not uncommon ; 



ISpool Col. 



Characters approaching C. onyx. Tablet 1362 contains the 

 specimen in U. unguis. 1363, 1 sp. nearly smooth, singular!^ 

 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 striated, 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, 

 4 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 attachment. 1379, Pisania hsemastoma, with 

 July 1856. aa 



