452 THE OPERCULUM. 



example of this form), having enabled me to understand in 

 what manner those opercula which appear to be the most 

 complicated in their structure increase in size and thickness, 

 I shall proceed to describe its mode of growth. A compari- 

 son with those of the other Trochi leaves little doubt that 

 they are all formed and increase in size on the same principle. 



The disk on the back of the foot of the animal to which 

 the operculum is attached, shows three very distinct parts, 

 viz. 1st, the muscle by which the operculum is affixed, which 

 is semilunar, and occupies nearly the whole of the anterior 

 half of the disk, having its hinder edge thin and membrana- 

 ceous ; 2nd, an elongated triangular fleshy band, on the right 

 side of its anterior part, which is separated from the muscle 

 by a deep groove ; and ord, the back part of the foot, which 

 is raised to the level of the muscle by an elevated border, 

 attached in front to the membi'anes placed along the sides of 

 the body of the animal, and is marked with minute concen- 

 tric wrinkles, originating round a longitudinal fissure on its 

 hinder edge, and becoming more and more transverse as they 

 approach nearer to the muscle. This part of the disk merely 

 forms a bed for the operculum to lie upon, without any at- 

 tachment ; it is semicircular, and is prolonged into a narrow 

 process extending up the right side of the triangular fleshy 

 band. If the inner side of the operculum be examined, it 

 will also be found to exhibit three parts, differing from each 

 other in colour and surface, but agreeing exactly in form 

 with the three parts described as found in the disk : 1st, the 

 scar of the muscle, which is green, and occupies nearly the 

 anterior half of the operculum : 2nd, on the left of the scar 

 (that is, when the operculum is turned with its inner side 

 towards the observer), a triangular pale brown spot, occupy- 

 ing the margin of the extremity and a triangular portion of 

 the last volution ; and 3rd, a black and polished posterior 

 portion. 



There can be no doubt, as these divisions of the opercu- 

 lum agree in shape with the three parts observed in the disk, 

 that they are severally deposited by them ; and on further 

 examination it appears that the volutions are in fact formed 

 of three coats, each deposited by one of these parts. The 

 new layers are first added to the extremity of the whorls by 

 the small triangular fleshy band placed on the right side of 

 the muscle. Afterwards, as the operculum is moved round 

 in order to present a new end to the influence of the fleshy 

 band, the newly formed part is covered by a black coat, 

 secreted by the process of the back of the foot which ex- 

 tends up the right side of the band. Both these layers are 



