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. 
]. 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 Ampullariade, or the spiral operculum 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 im 
the flat 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- 
ritelle and Cerithia, have also an operculum with many whorls 
which very gradually increase in size ; while the Littorine, Nerite, . 
and Natice, 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 
