THE OPERCULUM. 449 



disk to which they are attached, the remainder being formed 

 by a membranaceous flap attached to the hinder edge of the 

 muscle. Along the whole anterior edge of the muscle is a 

 ridge, separated from it by a deep groove, which evidently 

 secretes the additions to the operculum ; in one specimen I 

 thought that I could observe the edge of this part attached 

 to the ridge, in like manner as the edge of the periostracum 

 is adherent to the mantle of many bivalve shells. 



The operculum of the Naticas is formed on the plan just 

 described ; but its inner surface is covered in addition with 

 a smooth pellucid coat, resembling the inner coat of the 

 subannular opercula, and forming a curved tubercle over the 

 nucleus. This coat is marked with two muscular scars, the 

 one lanceolate and central, and the other anterior, linear, and 

 only separated from the first by a narrow polished band ; the 

 latter is extended to the back of the tubercle, where it forms 

 a deep oval impression. The adductor muscle occupies the 

 anterior half of the disk, its middle part, which produces 

 the central scar, being of a dark colour, while its front edge, 

 giving rise to the anterior, is white : it is furnished with a 

 thin membranaceous band posteriorly, which is broadest on 

 the right side ; and the remainder of the disk is formed of a 

 thick semioval elevated fleshy flap, quite distinct from the 

 back of the foot. 



The opercula of some shells which have plaits on their 

 pillar, are very thin, and are furnished with a moveable flap 

 on the left side of their anterior margin, which passes over 

 the plaits. I first observed this in the common Tornatella, 

 and afterwards in Turbo pallidus of Montagu (the genus 

 Odostomia of Dr. Fleming), and have since verified it in 

 Pyramidella. The subannular operculum of Turbinella cor- 

 nigera has a notch on the middle of its anterior margin, and 

 a plait running from the nucleus ; but in this case the flap 

 is not moveable. 



The opercula of the genus Neritina agree with those just 

 described, in their ovate form, and in being composed of few 

 and rapidly enlarging whorls ; but they appear to be entirely 

 formed of shelly matter, and are marked both externally 

 and internally with very minute concentric lines of growth. 

 Like the operculum of Littorina, they are not covered with 

 any internal or external deposit ; but they have several 

 peculiarities. The outer edge of their whorls is furnished 

 with a broad flexible margin ; and on the inner side, below 

 the spire, are two diverging processes, the longer of which is 

 curved and forms a tooth, placed near the left end of the 

 anterior edge, which some have supposed to act as a kind 



G (i 



