UNIVALVE SHELLS. 



485 



Fig. 100. 



Fig. 101. 



Opercula are annular when the whorls of which it consists 

 encircle a central nucleus, and are themselves circular. They 

 are subannular when the nucleus is 

 lateral, but with the whorls still form- 

 ing complete circles : all others are 

 spiral. 



An operculum is multispiral when it 

 is formed of numerous narrow volu- 

 tions with the nucleus near the centre : 

 it is simply spiral when the nucleus is 

 lateral, the spires increase rapidly in size, and are few in 

 number (Fig. 100) : unispiral when there is only one volu- 

 tion, as in Nerita ; and un- 

 quiculate when the apex is 

 terminal, and the whorls 

 half encircle it, always in- 

 creasing in diameter and 

 size as they follow each 

 other. (Fig. 101. a the 

 outer, and b the inner sur- 

 face.) 



The operculum is con- 

 fined to pectinibranchial 

 and one or two genera of 

 pulmonated Mollusca. The 

 great bulk of the pulmo- 

 nated order have no oper- 

 culum, but, previous to 

 hybernation, many of them 



form a covering for the aperture of their shells, which has 

 been sometimes described as an operculum. It is now called 

 the epiphragm. One genus of the Pulmonifera (Clausilia) 

 possesses a peculiar appendage for closing the aperture, 

 which was first described by Miiller. " It consists of a 

 spirally-twisted, thin, shelly-plate, inclosed in the last whorl 

 of the shell, and attached to the columella by an elastic 

 pedicle. When the animal is retracted within its shell, this 

 plate nearly covers the aperture at a little distance within 

 the mouth, and coming in contact with a transverse plait on 

 the outer lip, leaves only a small canal formed between the 

 outer plait and the posterior angle of the mouth, and some- 

 times an elongated longitudinal plait on the inner lip. When 

 the animal wishes to protrude itself, it pushes the plate on 

 one side into a groove situated between the inner plait and 

 the columella, where it is detained by the pressure of the 

 body of the animal, leaving the aperture free, and when the 



