478 Mr. J. E. Gray on the Operculum 



In the paper before referred to, I stated, "The operculum agrees 

 with the valve of shells in being developed on the embryo while 

 included in the egg, and in increasing in size by the addition of 

 new matter round the circumference of the base of the cone of 

 which they are formed. They also agree in the cone being some- 

 times simple and straight, and sometimes curved into a spiral 

 form." The principal difference between the operculum and the 

 valve or shell of the Gasteropods consists — 



1. In the operculum having no cavity, the cone of which it is 

 formed being either very much depressed, so as to become nearly 

 flat or even concave, as in the annular or subannular operculum ; 

 or very much compressed, forming only a spiral riband, as in the 

 spiral operculum. 



2. The operculum of by far the greater number of Gasteropods 

 is only formed of animal matter, so that the operculum is as if 

 formed entirely of what constitutes the periostraca or drap marin 

 of the shelly valves ; but the shells of some Gasteropods, as that 

 of the Aplysia, Bulla, and of some land mollusks, and the valves 

 of some bivalves, as Lingula, have only a very, thin shelly in- 

 ternal layer, strengthening the thick periostraca ; on the other 

 hand, many opercula, like the generality of shells, have a shelly 

 coat deposited on the inner side of the horny or periostracal coat, 

 and others have the outer surface of this part, like Cypraa and 

 some other genera of shells, covered with a shelly coat. 



The absence of a cavity is a difference only of degree, for the 

 valves of some Gasteropods, as Umbrella for instance, are so flat 

 as to produce no cavity, and thus greatly resemble the annular 

 opercula of Ampullaria and Paludina, as the flat valves of some 

 Calyptrce are like the spiral opercula oiLittorince ; but the greatest 

 resemblance is to be observed in the small flat valves of Gryphcea, 

 Exogyra, Chama, and other genera of bivalve shells which are 

 attached by one of their valves. These valves are often quite as 

 flat and destitute of any cavity as the operculum of any Gaste- 

 ropod ; and it is to be remarked that these valves exactly resemble 

 a spiral operculum in shape, the remains of the ligament forming 

 a spiral mark on the outer surface, showing how the valve has 

 rotated on the body of the animal as the operculum rotates on 

 the foot of the Gasteropods. 



Having thus shown the reasons which induced me to regard 

 the operculum to be a modification of the other or shelly valve 

 of a Gasteropod mollusk, I shall now proceed to show why I have 

 been induced to believe that it is analogous to the second valve 

 of a bivalve. 



In the Philosophical Transactions for 1833 I remarked, "A bi- 

 valve shell is composed of a dextral and a sinistral valve united 

 together by a ligament. When the two valves are separated and 



