16 COLUMELLAR MUSCLE AND OPERCULUM. 



sub-central, somewhat like the operculum of Paludina. Tlie 

 examination of the restoration of the lost half of the operculum 

 of the Fusus before referred to has solved the difficulty, and I 

 have no doubt that one of these animals had by some accident 

 lost its operculum, and that it had gradually restored it; com- 

 mencing, as in the case of the restored part of the operculum of 

 the Fusus, by a small nucleus in the centre of the opercular 

 mantle, on the back of the foot, and gradually adding new Ia3^ers 

 around the edge of it, until it formed an annular operculum 

 nearly of the size of the original, but differing from it in shape, 

 being less acute in front and nearly similar in form at the two 

 ends. A more minute examination has strengthened this theory, 

 for the operculum of this specimen is less regularly developed 

 than is usual in the annular operculum of the kind, and is much 

 thinner than the normal operculum of the genus, as is the case 

 in both these particulars with the restored part of the operculum 

 of the Fusus. 



u This change in the formation of the operculum when it is 

 reproduced is just what might have been expected. The animal, 

 when it has to form the operculum at its birth, begins its forma- 

 tion at the tip, and increases its size, as the animal requires a 

 larger operculum for its protection, by the addition of new layers 

 to the outer edge of its larger and last-formed end ; but when it 

 has to reproduce this organ, the opercular mantle having reached 

 a certain size, it proceeds to cover its surface with a new pro- 

 tection in the most easy and rapid manner, and, commencing 

 from a more or less central spot on the surface, enlarges the 

 surface covered by adding new matter to the entire circumfer- 

 ence of the first-formed part ; it continues this process without 

 waiting to making the operculum as thick and solid as the one 

 which was lost, until it readies the size of the original, moulding 

 itself on the opercular mantle, and adapting its form to the form 

 of the throat of the aperture of the shell which it has to close. 

 The change of form in the front of the restored and mended 

 operculum is caused by the parts being moulded on the existing 

 opercular mantle consequently they have not the narrow front 

 part which is found in the normal form caused by that part 

 having been formed when the animal had this part of a small 

 size; and as it increases in size the whole opercular mantle 



