12 GASTEROPODA. 



spiral shell turns on its imaginary axis, causing 

 the muscles to move down the inner surface of the 

 aperture. 



6. The operculum is moulded on the opercular 

 mantle, and is often lined internally with a shelly 

 coat ; and sometimes, as in certain shells, it has its 

 outer surface covered with calcareous matter, de- 

 posited by some special development of the oper- 

 cular mantle destined for the purpose, as is the case 

 with the cowries and some other shells. 



From these observations it appears that the oper- 

 culum has all the characters of the appendage of the 

 animal which has been usually considered as the 

 shell of univalves, and the valves of bivalves. 



7. That as the valves of bivalves are always 

 twisted in opposite directions, so that each bivalve 

 shell is composed of a dextral and sinistral valve 

 united together by a ligament, so the operculum of 

 a shell is always turned in the contrary direction to 

 that of the shelly valve of the animal to which it be- 

 longs, the dextral shell having a sinistral operculum, 

 and vice versa. Thus the position of the nucleus of 

 the annular operculum, or the spire of the spiral 

 operculum, is always twisted in an opposite direction 

 from that of the shell to which it belongs, as is the 

 case with the two valves of a conchiferous animal. 

 This is easily observed by comparing the position of 

 the nucleus of the dextral and sinistral genera of 

 Ampullaridae, or the spiral operculum of a sinis- 

 tral malformation of a gasteropod, with that of one 

 of the normal form. I may observe that, as is 

 the case with spiral shells, when the shell is turned 



