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 Loven^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 Neritinida, 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, " II imite parfaitement en cela le second 

 battant des coquillages bivalves.^^ — Fby. 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. 31 



