12 
GASTEEOPODA. 
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 
Ampullaridse, 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 
