454 THE OPERCULUM. . 



regarding as opercula, but the structure of which has not 

 previously been examined in detail, I am inclined to add 

 two other bodies belonging to ctenobranchous molluscous 

 animals, which have hitherto been generally regarded as ano- 

 malous. The first of these is the support, as it was called 

 by its discoverer M. de France, or under valve, as it has since 

 been regarded by some English conchologists, of the genera 

 Capulus and Hipponyx. I am induced to regard this body 

 as analogous to the opercula of other spiral shells, because, 

 on an attentive examination of the animals, I find that it is 

 attached in the same situation and not on the under side of 

 the foot, as most conchologists have supposed ; the foot being 

 folded on itself, and the walking disk of other Gasteropods 

 being in these animals (which never move from the place of 

 their first attachment, and consequently require no such ex- 

 pansion), represented by a few crumpled folds placed between 

 the part to which the shelly plate is attached and the head. 

 In this idea I am further confirmed by a somewhat similar 

 structure of the foot in the genus Vermetus, where the back 

 of that organ represents a truncated cylinder, filling up and 

 closing the mouth of the tubular shell. This foot is crowned 

 by a horny operculum, and the walking disk is reduced to a 

 narrow flat band, passing along the front of the cylinder, 

 which band is in some species terminated by two conical pro- 

 cesses, situated between that part and the base of the head : 

 the processes have been described as tentacula, which they 

 resemble in form. The shelly plate or operculum of Capulus 

 is formed of concentric shelly laminae, with a nearly central 

 nucleus, and differs from all other opercula at present known, 

 in being immediately attached, by its outer surface, to other 

 marine bodies, like the lower valves of the Oyster and of 

 Crania, and thus forming the medium by which the animal 

 is retained in its place. The mouth of the shell being nearly 

 as large as the cavity, the adductor muscle, as in other shells 

 of this form, is divided into two broad bands, forming a 

 horse-shoe-shaped, posterior, submarginal, muscular scar, and 

 the operculum is marked with a similar impression. 



The second body to which I refer is the vesicular append- 

 age, placed on the back of the hinder part of the foot of the 

 animals belonging to the genus Ianthina,* which appears to 



* Cuvier at the time of publishing his Anatomy of Mollusca, appears to 

 have entertained the same theory, for he there properly describes this body 

 as attached to the hinder part of the foot, a little below the usual place of 

 the operculum ; but in his Animal Kingdom he seems to have abandoned it, 

 and describes the animal as having no operculum, but having a vesicular 

 organ under its foot. — Regne Animal, ed. 2, torn. iii. p. 84. 



