of Gasteropodous Mollusca. 477 



as in Atlanta , sometimes very reduced; on its middle the byssus 

 is secreted, whose filaments when present are always united into 

 a disc-like operculum. 



" The side part of the lobe is sometimes produced into cirri, 

 as in Rissoa, Lacuna, &c, sometimes into a large extensible mem- 

 brane capable of covering the whole shell." — Loven, Ofvers. 

 Kongl. Vetensk. Akad. Fork. 1847. 



In a paper on the Structure of Shell in the Philosophical 

 Transactions for 1833, 1 showed with considerable detail that the 

 operculum of the Gasteropodous Mollusca, like the shelly valve 

 of those animals, 



1. Is developed on the embryo long before it is hatched. 



2. That it is placed on and covers a peculiar part of the body, 

 which bears the same relation to it as the part of the body called 

 the mantle bears to the part usually called the shell of these 

 animals ; and it is formed and increased in size by an opercular 

 mantle in the same way as the shells are. 



3. That the operculum is more or less conical, and is increased 

 in size by the addition of new matter to the inner surface, and 

 especially to the surface near the margin ; the new matter either 

 forming more or less complete rings round the nucleus (or first- 

 formed part), when they are called annular, and are homologous 

 to the simply conical shell, as the Patella; or else the new matter 

 is deposited almost entirely on one edge of the nucleus, when the 

 operculum forms a more or less elongated cone, which, when long, 

 is spirally twisted round an imaginary axis (like a spiral shell), 

 the broad part of the cone being next the edge of the opercular 

 mantle which secretes the new matter for enlarging its size, as 

 the mouth of the shell is on the outer edge of the mantle of the 

 univalve shell. 



4. That the operculum is attached to the animal by means of 

 one or more muscles, which, as in the bivalve shell, pass from 

 the larger valve or shell to the smaller one or operculum. 



5. The operculum as it increases in size is gradually moved 

 on the end of the muscle ; the many-whorled operculum of the 

 Trochi revolves as many times on the end of the muscle, as the 

 many-whorled spiral shell turns on its imaginary axis. 



6. The operculum is moulded on the opercular mantle, and 

 is often lined internally with a shelly coat like a shell ; and some- 

 times, like the shell of the Cowries, it has its outer surface co- 

 vered with a shelly coat deposited by some special development of 

 the opercular mantle especially destined for the purpose, as is the 

 case in the Cowries and some other shells. 



From these observations it would appear, that the operculum 

 has all the characters of the part of the animal which has been 

 usually considered as the shell. 



