THE OPERCULUM. 455 



assist in floating the animal on the surface of the sea, and 

 probably also in supporting the eggs after the death of the 

 parent. This float, as it has been called, I am inclined, from 

 its being situated in the same position as other opercula, to 

 regard as analogous to those bodies in the neighbouring 

 genera. 



In the "Medical Repository" for 1821, I first called the 

 attention of conchologists to the importance of the characters 

 furnished by the operculum for the distinction of genera and 

 families ; and this subject, although almost neglected in this 

 country, has since been pursued with great assiduity by 

 M. de Blainville and other French conchologists. I have 

 fortunately had an opportunity of examining, either in the 

 cabinet of the British Museum or in the Continental collec- 

 tions which I have visited, the animals of the greater number 

 of genera of shells, and have been thereby enabled to deter- 

 mine that the form and structure of their opercula offer 

 some of the most constant characters for the distinction and 

 arrangement of families and genera ; while, on the other 

 hand, I have convinced myself that systematists have been in 

 the habit of placing too much reliance on the mere fact of 

 their presence or absence as a family character, inasmuch as 

 that circumstance alone will scarcely prove of generic im- 

 portance. Thus in the genus Voluta, the animals of the 

 eight or nine species which I have examined are all destitute 

 of opercula, except Voluta musica, which has an operculum 

 of moderate size. The Olives and Mitres are in the same 

 predicament, most of the large species being destitute of 

 opercula, while the smaller species of both genera are fur- 

 nished with rather large ones, as may be easily seen in speci- 

 mens of Oliva eburnea, O. zonalis, or Mitra striatula, in 

 which the animals have been dried ; and shells in this state 

 are not uncommon in collections. The species of Cones 

 offer in this respect the same variations. These observations 

 will explain the apparent contradictions of describers, and 

 the frequent controversies that have taken place as to whether 

 these and some other genera have or have not opercula. 

 That their presence or absence is not a family character may 

 be inferred from all the genera of Buccinidae being provided 

 with them except Harpa and Dolium. And this leads me 

 to remark, that many genera and species which have very 

 large mouths, in comparison with others of the family to 

 which they belong, are destitute of, or have very small, oper- 

 cula, whilst the others have moderately sized or even large 

 ones. Thus the wide-mouthed Cones, as, for example, Conus 

 geographicus, have no operculum, whilst the other species 



