14 



a collar which projects for a little distance, sheathing the 

 unattached marginal regions of the operculum. This 

 " opercular lip " is deepest on the posterior border where 

 there is a greater width of unattached operculum. When 

 creeping, the operculum is arranged, as we have seen, 

 transversely across the foot. The anterior margin in 

 this position is the internal border, the posterior margin 

 the external border when the operculum is applied to the 

 shell mouth. 



Fig. 2a. 



If the operculum is removed from the foot and the 

 attached or ventral surface examined, a very different 

 system of striae will be seen. The operculum is, as a 

 matter of fact, composed of several layers, and the 

 markings on the superior and inferior surfaces are 

 therefore quite independent of one another. The area 

 of attachment, as already mentioned, is situated towards 

 the anterior margin, and entirely to one side, therefore, 

 of the nucleus. This area is marked by about ten bands 

 (in adult specimens), arranged concentrically, but with 

 only part of any band showing, the remainder being 

 outside the area of attachment. Furthermore, these 

 bands are arranged in an imbricating manner (Text-fig. 

 2a, b), each one nearer to the centre overlapping its 

 more marginal neighbour. The area outside the 

 region of attachment is covered with a somewhat 

 glossy horny layer, which hides the concentric bands 



