480 Mr. J. E. Gray on the Operculum 



2. That^ like the univalves, these valves are sometimes re- 

 versed. 



3. That the valves move on the body of the animal as the uni- 

 valve shells do, to allow the deposition of new shelly matter to 

 the margin ; the position of the hinge on the margin being gra- 

 dually altered to allow of this motion. 



The operculum agrees with the second valve of a bivalve in all 

 the preceding particulars. 



1 . 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 mollusk. This is easily 

 observed by comparing the position of the nucleus of the dextral 

 and sinistral genera of Ampullariadce, or the spiral opercuhim of 

 a sinistral malformation of a Gasteropodous mollusk with that 

 of one of the normal form , 



2. These valves are sometimes reversed, as in the instances 

 above cited. 



3. The operculum moves on the foot as the valves do on the 

 body, and they always bear the same relative situation to the 

 valves as the valves do to each other. In the ' Synopsis ' of the 

 British Museum for 1842, p. 56, when referring to the Phyto- 

 phagous Gasteropods, I observed, " Many of them have a spiral 

 operculum or lid which is attached to the back of the hinder part 

 of the foot of the animal. This operculum turns round back- 

 wards on the apex of its spire as it increases in size by the addi- 

 tion of new matter to the edge of its last whorl, so that this edge 

 is always in the same position in the mouth of the shell.^' 



The two valves of the bivalve move at the same rate, and there- 

 fore the lower attached valve of the Chama, which often has the 

 apex produced into a conical tip like the spire of a univalve, and 

 marked like it with a spiral groove formed by the remains of the 

 cartilage, similar to the suture of the whorls ; and the flat valve 

 with its simple spiral groove has the same number of twists in 

 the fiat and the elevated spire of the two valves. The same ap- 

 pears to be the case with the opercula of univalves, as the num- 

 ber of volutions of the operculum appears to bear a relation to 

 the number of whorls in the shelly valve. Thus all the shells 

 which have many gradually increasing whorls, as the Trochi, Tur- 

 ritellcB and Cerithia, have also an operculum with many whorls 

 which very gradually increase in size ; while the Littorina, Nerita, 

 and Natica, which have a few more or less rapidly increasing 

 whorls, have an operculum of that character which have hence been 

 called neritoids; but there appear to be some exceptions to this 

 rule, which require examination. 



In addition to these similarities it may be observed, that the 

 operculum, like the two valves of a bivalve, is united to the valve 



