The Nautilus. 



Vol. XXX. JANUARY, 1917. No. 9 



NOTES ON OPERCULUM EVOLUTION. 



BY CHARLES HEDLEY. 

 Australian Museum, Sydney, N. S. W. 



Conchological text books describe various styles of opercula 

 such as the concentric, lamellar, unguiculate, multispiral, pan- 

 cispiral, or articulate. Yet little, if anything, has been said on 

 the inter-relation of these various forms. Woodward and 

 Fischer even warn their readers against trusting to opercula for 

 guidance in classification. 



Dr. J. E. Gray had a clearer idea of the homologies and tax- 

 onomic value of the operculum than many of his successors. 

 In 1833 he showed that spiral opercula can only grow by rotat- 

 ing backwards on their axes, just as a spiral gasteropod shell 

 rotates backwards with regard to the columellar muscle. He 

 maintained that shell and operculum were morphological twins 

 representing the right and left valves of a pelecypod shell. 

 This hypothesis he supported by the correspondence in the 

 appearance of the operculum early in embryonic life, by the 

 right-hand spiral of the shell reflected in the left spiral of the 

 operculum, or vice versa, by structural resemblance and by the 

 frequent mutual gain or loss in solidity or complexity. The 

 muscle between the columella and operculum may be compared 

 to the adductor muscle of a bivalve, while the operculigerous 

 lobe answers to the mantle. 



Among recent writers Thiele supported this view ; on the con- 

 trary, Huxley disputed Gray's homology and asserted that the 

 gasteropod operculum was equivalent to the bivalve byssus. 



