UNITALTE SHELL AND ITS OPERCULrM. 205 



G-asteropoda haviug a dextral shell commenciug with a sinistral 

 nucleus, the spiral nucleus of the operculum is also sinistral, and 

 in the spiral-shelled Pteropoda, the gyri of both shell and opercu- 

 lum are alike sinistral. The legitimate deduction in this latter 

 case is that the body-valve of the Pteropod, though sinistral, is the 

 equivalent of the dextral body-valve of the Gasteropod ; and both 

 we assume to be answerable to the left valve of the Conchifer. It 

 is sometimes stated that Atlanta affords an exceptional example of 

 a dextral shell with a dextral operculum* ; but this I can confi- 

 dently affirm is not the case ; the operculum of^i^/^n^a is sinistral,' 

 like all normal opercula. 



As relates to the umbones of bivalves and their homologues in 

 the shells of Gasteropoda, the spii'al nucleus of both body-valve 

 and operculum is very significant. From the examination of 

 numerous species of the operculated genera of Heteropoda captured 

 from time to time in the Atlantic and the Pacific, I discovered that 

 in all those shells possessing a promiuent spiral nucleus, the cor- 

 responding little point of the operculum was always beautifully 

 curved and proportionately well expressed ; but when the nucleus 

 of the shell was involute, the nucleus of the operculum presented 

 a most indefinite and irregular appearance, though still obviously 

 disposed in a spiral fashion. This latter case is illustrated in the 

 genus Oxy gyrus (Benson), characterized by having a more or less 

 horny shell with an involute nucleus. 



The study of the development of the Conchifera, and the nice 

 examination of the relations and connexions of the mantle, tend 

 to show that that part of the latter which lines the right valve is 

 quite distinct from that which Kues the left ; and the primitive 

 distinctness of these two portions is permanently exemplified by 

 their homologues in the operculated Gasteropoda. 



Any one who will examine the operculigerous lobe in the genus 

 Turho, cannot fail to recognize the idea of an opercular mantle, the 

 source of that remarkable deposit of shelly matter which so distin- 

 guishes Turho, Phasianella, and other genera of the same family. 

 The free margins of this opercular mantle are often ample enough 

 to meet over and conceal the operculum completely, and they may 

 be traced in continuity like those of the body-valve mantle round 

 what I have been induced to call the "pedicle,''^ which encloses the 

 retractor or adductor muscles, and connects each mantle respect- 

 ively with the body of the foot and the investment of the visceral 

 mass. 



* Woodward's Manual of MoUusca, p. 201. 



