III.] INTRODUCTION. Iv 



mollusk from the attacks or intrusion of other animals. 

 It is often formed on the same plan as the spire of the 

 shell, but it differs from the latter in being nearly always 

 compressed instead of tubular. The only exception to 

 this rule of which I am aware, as far as regards European 

 Mollusca, is that of Zanclea, where the operculum is 

 pyramidal. It has, indeed, been stated that the opercu- 

 lum o£ Adeorbis subcarinata is cellular ; but the supposed 

 operculum of this rather common shell belongs to the 

 Foraminifera and is the Spirillina perforata of William- 

 son. The mistake arose from the shells and Forami- 

 nifera having been found by the late Mr. William Clark 

 in the same parcel of dredged sand ; and as the latter 

 exactly fitted the mouths of the former, he concluded 

 that the Foraminifera were the opercula of the shells. 

 The original specimens are now in my collection. A few 

 of our native Mollusca, as well land as fresliAvater and 

 marine [e. g. species of Cyclostomay Neritina, Bythiniu, 

 and Phasianella), have calcareous or shelly opercula. 

 The opercula of other kinds are horny and usually thin. 

 The operculum of Neritina and Jeffreysia is furnished 

 with an excentric process, or apophysis, which enables 

 it to fit more closely into the shell, like the bolt of a lock 

 into the box. In most cases the operculum is spiral; 

 but in Paludina, Phasianella, and a few other genera it 

 is concentric. The whorls on some of the horny opercula 

 nearly correspond in number with those of the shell, 

 being multispiral in Trochus and paucispiral in Littorina ; 

 but this rule is not universal. There is a difference of 

 opinion among physiologists as to the mode in which the 

 operculum is formed. Some consider the mantle as the 

 organ of secretion, others the foot, while according to a 

 few it is formed by the glands of a special organ called 

 the operculigerous lobe. Adanson and lately Dr. Gray 



