270 MAZATIAN UNIVALVES 
commencement of its deck, which it continues from it in a 
regular curve. The marginal lip also is very short, not cover- 
ing the spire as in C. nivea, &c., but leaving it for a long time 
very conspicuous. ‘The shell in its early decked stage has 
much the appearance of a Neritina. The deck bowever is 
extremely thin, always displaying strie of growth. It soon 
develops a central sinus, leaving the margin arcuated, not 
angulated. From this regular spiral growth, the transition is 
easy, through other species, to the form Trochita. The adult 
shell has normally a deck margin of the form ~A, one side 
of the brace being longer than the other. The point develops 
a spiral line to the apex. Sometimes however this point is 
rounded ; each sinus and lobe may be developed at the expense 
of the rest ; and in one specimen, the outer lobe being pointed 
while the middle point is rounded, the margin assumes the 
form of an irregular inverted brace wv~. The ordinary 
colour is a yellowish white variously striped, spotted or mottled 
with lustrous chesnut or dark brown. Rarely the whole shell 
is dark brown, most rarely pure white. The red tints obsery- 
able in the Ch “ili and Atlantic specimens have not been found in 
the Gulf district : they are not mentioned however in Dillwyn’s 
description, and perhaps are due to the fading of the brown, 
most of the W. Indian specimens in collections being dead. 
Those brought by Mr. D. Dyson from Honduras are coloured 
like the Mazatlan specimens. The external markings, on 
which Brod.’s species are founded, are extremely variable. 
Well developed specimens of hystrix and echinus are rare ; 
but intermediate forms between these and the common state 
with irregular crowded small vaulted spines, are abundant. 
The W. Indian specimens moreover go through the same 
changes of sculpture. The delicately grown shells, which are 
flatter and broader in proportion, are rough to the touch, but 
the spines are scarcely discernible without a glass. On coarse- 
ly grown shells, they are often not developed over part of the 
surface. The amount of spiral involution varies considerably 
in different specimens. (Comp. C. Calyptreiformis, Desh. in 
Lam. An. s. Vert. vol. vii. p. 647, no. 15.) The smallest 
specimen is *03 in length. 
The largest sp. measures long. 1°73, lat. 1°06, alt. °41. 
A convex sp. a Ps fd LUG U7 Pi Meat FAP py oi Os 
A flat Sp. 33 ” i 1 39 te 3 "26. 
An elongated sp. 8, “41, 1G: 
A convolute sp., apex to fr ont “68, 7 back 33. 
A straight sp. a 200 a eel 
