139 



developed in the different species. The shell of Troches seems' to exhibit 

 an exuberance of design in this respect, winch accompanies its pyramidal 

 form, and offers a generality of character by winch to distinguish it from 

 Turbo. In every species of Turbo, which is not absolutely smooth, the 

 sculpture, be it ribs, or tubercles, or scales, is arranged transversely, but 

 in Trochus it is mostly carved longitudinally or obliquely, in patterns more 

 varied and more fanciful. In T. calcar the whorls are armed with promi- 

 nent tufted tubercles, in T. miff ids with vaulted scales, in T. nndosus they 

 are bordered with a waving rib, like a twisted cable, in T. olivacens carved 

 with oblique slightly waved ridges, wliilst the T. gemmosus, anmdatus, and 

 others, are studded with various designs of beading. 



The Troclii are numerous in species, and widely and abundantly distri- 

 buted; they occur on our own coast, also in the Mediterranean, and in 

 all tropical latitudes. 



For List of Species see Appendix. 



Figure. 



Trochus modestus. PI. 13. Fig. 67. Shell, showing its pyramidal 

 form. — From Mr. Cuming's collection. 



Genus 9. TROCHISCUS, Sowerby. 



Animal ; ? 



Shell ; depressly orbicular, somewhat discoid, largely and deeply 

 umbilicated, rather solid, smooth, pearly within, margins dis- 

 joined, lip simple. Operculum horny, internally very smooth 

 and shining, externally set with circular rows of fibrous flakes 

 curled over toioards the centre. 



The solitary species which has been erected into a genus under the name 

 of Trochiscus is certainly a shell of very anomalous character. It may be 

 described as a compound of Turbo, Trochus, Solarium, and Rotella, par- 

 takin 0, most of the typical structure of the first of these genera, but scarcely 

 comprehended within the limits assigned to it by Lamarck. The shell is of 

 a heavy Hofella-like substance, smooth on the outer surface, and, as in 

 Solarium, the umbilicus penetrates inwardly to the apex. The operculum, 

 on the other hand, is more like that of Delphimda, with the addition of 

 the prominent circular rows of curled flakes noticed above. 



It is a native of New Holland. 



t2 



