420 On the Reproduction of a lost part of an Operculum. 



operculum of the Fusus before referred to has solved the diffi- 

 culty, and I have no doubt that one of these animals had by 

 some accident lost its operculum, and that it had gradually 

 restored it; commencing, 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 layers round the edge of it, until it formed an annular oper- 

 culum 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. 



This change in the formation of the operculum when it is re- 

 produced, is just what one might have expected. The animal, 

 when it has to form its 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 re- 

 produce this organ, the opercular mantle having reached a certain 

 size, it proceeds to cover its surface with a new protection 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 circumference of the first-formed part ; it 

 continues this process without waiting to make the operculum 

 as thick and solid as the one which was lost, until it reaches 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 moves forward, leaving the small tip 

 of the operculum free, and useless to the animal, and therefore 

 not necessary to be reproduced when the operculum is re-formed 

 in the adult age of the animal. 



In the British Museum collection there is also a specimen of 

 Cominia maculata with the operculum almost entirely repro- 

 duced, with the same alteration of the general form and position 

 of the nucleus. These mended or reproduced opercula are 

 always known from the normal operculum of the animal by being 

 more or less irregularly formed and thinner in consistence. 



