482 On the Operculum of Gasteropodous Mollusca. 



culum together are homologous to the two valves of a Conchi- 

 ferous moll ask. 



I was also led to believe that the normal or typical form of 

 Mollusca is to be protected by two valves or shells, and I was 

 strengthened in this impression by the discovery that several 

 mollusks which have no shell in their adult state, as the Doridee, 

 &c., have their newly hatched young covered with two shelly 

 valves which afterwards fall off. 



With this idea, in the ^ Synopsis ^ of the British Museum for 

 1842, p. 50, I observed, " By far the greater number of these ani- 

 mals (Mollusca) are provided with two of these shells or valves, 

 which are often nearly alike in size and form, and are hence called 

 bivalves, as the shells of the Conchifera, where one of the valves is 

 placed on each side of the body and they are united together by a 

 ligament. In others, as those of the Brachiopods, the two valves 

 are separate, one on the upper surface or back, and the other on 

 the under surface of the body. In others, as in the shells of 

 Gasteropods, the two valves are so unequal that the smaller 

 merely acts as a lid to close the mouth of the larger one when 

 the animal is retracted into it ; hence it has been called an opier- 

 culum. This smaller valve or operculum is generally cartilagi- 

 nous, either wholly formed of animal matter, or strengthened by 

 a quantity of calcareous matter deposited on one or both of its 

 surfaces ; sometimes this valve is altogether wanting, especially 

 in those genera which have an expanded mouth compared with 

 the size of the remaining shell. In the bivalve Conchifera and 

 Brachiopoda the two valves are usually nearly equal-sized, and 

 regular in position. On the contrary, in the Gasteropods the valves 

 are unequal, and placed more or less obliquely with regard to the 

 axis of the elongated body of the animal." 



If this theory is correct, the operculum should afford an im- 

 portant character for the distinction of families and genera ; and 

 this has proved to be the fact. 



In 1821 1 first drew attention to the very good character which 

 it afforded, not only for the distinction of genera, but also for 

 the division of the genera into larger groups. In my papers pub- 

 lished in the Zoological Journal and in the Philosophical Trans- 

 actions, I collected together the results of my observations on 

 their structure, formation and growth, and their importance to 

 the oeconomy of the animal. More recent examinations have 

 only strengthened my conviction, that they afford quite as import- 

 ant characters for the division of families and genera as the shell 

 of the Gasteropods themselves, and that to neglect them in the 

 description of the genus or species is quite as rational as to de- 

 scribe only the single valve of a bivalve shell. If this is the case, 

 no specimen of an operculated univalve, which is not accom- 



