270 MAZATlAN UNIVALVES 



commencement of its deck, ^^■llich 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 JS^eritina. The deck however is 

 extremely thin, always displaying strife 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 rvA>n, 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 v-^'YVJ. The ordinary 

 colour is a yellowish white variously striped, spotted or mottled 

 with lustrous chesnut or dark brown. Earely the whole shell 

 is dark brown, most rarely pure white. The red tints observ- 

 able in the Chili and Atlantic specimens have not been found in 

 the Grulf 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 intci'mediate forms between these and the common state 

 with irregular crowded small vaidted 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 mtliout a glass. On coarse- 

 ly grown sliells, they are often not developed over part of the 

 surface. The amount of spiral involution varies considerably 

 in different specimens. (Comp. C. Cal^-ptraiiformis, Desk, in 

 Lam. An. s. Vert. vol. vii. p. G17, no. 15.) The smallest 

 specimen is "03 in length. 



The largest sp. measures long. 1'73, 



A convex sp. ,, ,, \'\2, 



A flat sp. ,, ,, ^I'l, 



An elongated sp. „ "8, 



A convolute sp., apex to front 

 A straight sp. , 



