137 



short pedicles at the outer base, and the disk is short and thick, with 

 mostly a calcareous, sometimes a horny operculum. The number of species 

 is, however, very extensive, and the shells of these genera afford sufficient 

 characters to entitle them to be generically distinguished in the manner 

 commonly adopted. Much undue value has been set upon the composition 

 of the operculum, and a new division of the Lamarckian TrocM and Tur- 

 bines has been attempted, by tlu'owing the species of these genera together, 

 and re-arranging them according to the operculum, referring those in 

 wliicli it is horny to Troclms, and those in which it is calcareous to Turho. 

 Observation tends, however, to show that the operculum is a very subordi- 

 nate part of the species, and that in Trochus and Turbo, as in Natica, it 

 is sometimes horny, sometimes calcareous, without any corresponding differ- 

 ences in the shell or its animal inhabitant, to support the notion of its 

 indicating a difference of genus. 



The opercula of the Turbinacea furnish no characters Tor generic arrange- 

 ment, but are well worth observing on account of their variation in different 

 species. It is mostly of a solid testaceous substance, sometimes smooth, 

 sometimes granulous ; in some species it is deeply circalarly grooved, the 

 ridges being granulated or serrated, whilst in others it is composed of a 

 crowded tuft of club-shaped particles. In all these, the inner surface is 

 coated with a horny layer, and in the T. pica and one or two other species 

 the operculum consists of a horny lamina only, without any calcareous 

 deposit. Considerable variation will thus be found to occur in the oper- 

 cula of shells without any corresponding variation in the shell, and shells 

 of very different character have very often the same operculum ; the same 

 heavy, stony operculum which is common to Turbo is frequent in Troc/ms, 

 and the horny operculum of Troc/ms is to be met with in Turbo. 



It results from these observations that there can be no true generic 

 distinction between the genera of wliich we have been treating ; the TrocM 

 are to the Turbines among marine shells, what the Carocolla are to the 

 Helices among land shells, the animal in neither case presenting any 

 apparent difference ; but for the convenience of reference, and in order not 

 to subvert the names by which the species of this group have been so long 

 familiarly known, the Lamarckian genera may be adopted without prejudice 

 to the laws of classification. 



The Turbines are sometimes smooth, almost porcellanous, but mostly 

 spirally ribbed or grooved, ornamented with scales or lammaj. The inte- 

 rior layer and chief substance of the shell consists of mother of pearl, often 

 very iridescent and sometimes of a golden hue. The shells are remai'kable 

 for their symmetry of form and vivid admixture of colours, they are distin- 

 guished by their bold tubular growth, and wUl not stand pyramidally on 

 their base like the TrocJd. A complete monograph of the genus published 

 in the 'Conchologia Iconica' comprises sixty species, and shows their 



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