Ivi INTRODUCTION. [CH. 



have likened the operculum to the second valve of bivalve 

 shells ; but these do not appear to be homologous organs, 

 although equally serving to cover the body of the mollusk. 

 Calcareous processes, which answer the purpose of oper- 

 cula, occur in a genus of land snails (Clausilia), as well 

 as in marine bivalves belonging to the genus Teredo. 

 In the former case this process consists of a twisted plate, 

 which is not attached to the animal, but acts like a spring- 

 door in closing the interior of the shell ; and in Teredo 

 there are two such processes, each shaped like a spatula, 

 and attached to the body by strong muscles. Specimens 

 of Buccinum undatum and Fusus antiquus are sometimes 

 bioperculate ; and occasionally one of these opercula is 

 divided into two, or even three laminas, which are piled 

 upon each other, so as to give the specimen the appear- 

 ance of having three or four opercula. In some cases of 

 this malformation in Buccinum undatum, the two oper- 

 cula are too large to be contained within the shell, and 

 overlap each other ; but in others they are abortive and 

 widely separate from each other. They are found in 

 individuals of all ages ; and they appear to be congenital, 

 and not the result of accidental loss and renewal. In 

 one instance of malformation connected with this subject, 

 which fell under my observation, a deficiency, instead 

 of a redundancy, of opercular formation occurred, and 

 may have been caused by disease. A living specimen of 

 Fusus gracilis, which I procured last year in the Shetland 

 Isles, had no operculum or even the scar or trace of any 

 such process. The back of the foot, where the oper- 

 culum was placed in other specimens of the same kind, 

 was merely hardened by exposure. In Buccinum Hum- 

 phrey sianum the operculum is very small, and only covers 

 part of the aperture or mouth of the shell when the animal 

 withdraws itself. In some species of the genus Mangelia 



