50 THE OPERCULUM. 



That such an occurrence would most probably be rare, is easily 

 explained from its situation, as the operculum is protected by 

 the last whorl of the spire of the shell when the animal is 

 expanded, and by the mouth when it is contracted into the 

 cavity of the shell. 



" I have lately met with a very distinct example in a specimen 

 of Pusus in the British Museum collection. In this specimen 

 the apical half of the operculum has been broken off and the lost 

 part has been renewed by an irregular roundish process, nearly 

 of the size of the lost part, not quite as thick as the original 

 portion, and formed of rather irregular horny plates ; the smaller 

 or first-formed portion being in the centre of the broken line, 

 so that the restored part bears some similarity to the annular 

 operculum of a Paludina, This restoration is exactly like that 

 which would have taken place in a shell under similar circum- 

 stances, and is a further proof of the truth of the theory which 

 I have long advocated, that the o{)erculum is a rudimentary 

 valve, and is homologous to the second valve of the bivalve 

 mollusks. 



" In examining two specimens of Pleurotoma babylonica, pre- 

 served in spirits, with the opercula attached, I was much surprised 

 to observe that the opercula of the two specimens were exceed- 

 ingly difterent in structure and belonged to two distinct modifi- 

 cations of that valve, one being subannular, with the nucleus 

 apical, like the other species of the genus, and the other annular, 

 with the nucleus subcentral, somewhat like the operculum of 

 Paludina. The 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 grad- 

 ually 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 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 exami- 

 nation 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 

 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 



