of Gasteropodous Mollusca. 481 
of the univalve shell by muscles passing from one to the other, 
which by their contractions bring them together. The forms of the 
muscles which are used for this purpose differ in disposition and 
number according to the form of the mouth of the shell or the 
form of the valves. Thus when the valves are nearly circular, or 
the mouth of the univalve shell and the operculum moderate, the 
muscles form only a single group; on the contrary, when the 
valves are oblong elongate, or the mouth of the univalve shell 
very large, they form two groups of muscular fibres, one on each 
side of the valve or cavity of the shell and operculum. 
This attachment of the operculum is important ; for some con- 
chologists appear to have regarded the opercula of Gastero- 
podous Mollusca as analogous to the accessorial valves of certain 
bivalves, such as the genus Pholas, which they have remarked 
are formed in the same manner as the true valves, by the 
addition of new shelly matter to the edge. But these accessorial 
valves are never affixed to the animal by muscle, while the oper- 
culum, as above described, is attached to the animal, and is 
affixed to the other valve just in the same manner as the two 
primary valves of a bivalve are affixed to each other. 
Other conchologists, because the plug which passes through 
the sinus of an Anomia has been called an operculum or stopper, 
have regarded the operculum of Gasteropods as analogous to 
that substance, overlooking the fact that the plug of the Anomia 
is but a modification of the byssus, an excretion by which many 
molluscous animals more or less permanently attach themselves 
to other bodies, and has no affinity to a shelly valve; unless we 
adopt Professor Lovén’s theory above quoted, and believe the 
operculum to be a modification of the byssus. On the other 
hand, it may be observed, that the two valves of a Conchiferous 
mollusk are always united together by a ligament, while the 
opercula of the Gasteropodous Mollusca are always quite free 
from the shelly valves; but the importance of this peculiarity 
disappears when we consider that the two valves of the Brachio- 
podous Mollusca, which are so perfectly homologous to the valves 
of the Conchifera in other particulars, are always free and with- 
out any ligament ; and secondly, that the opercula of one fa- 
mily of Gasteropods, viz. the Neritinide, are furnished with a pe- 
culiar tooth-like process which enables them to move on the sharp 
inner lip of the larger valve, and greatly resembles the hinge of 
the valves on the Conchifera. Adanson, describing this kind of 
operculum, observes, “Il imite parfaitement en cela le second 
battant des coquillages bivalves.”— Voy. Senegal, 41. 
From these observations I was induced to believe the oper- 
culum of a Gasteropodous mollusk to be analogous to the shelly _ 
valve of the same animal, and that the shelly valve and the oper- 
Ann. & Mag. N. Hist. Ser. 2. Vol. v. dL 
