208 



OPIS. 



shell ; the under part of the body whorl flattened or concave, 

 umbilicate. O.Solaris. Mart. 173. f. 1/00, 1701. Indicus. lb. 

 172. f. 1697. 1698. It is probable, from the above description, 

 that Mr. Swainson intended to include Trochus agglutinans of 

 authors. (Genus Phorus, Montf.) Fig. 360. 

 OPERCULAR. Of, or belonging to, the operculum. A term 

 applied to the valves which compose the operculum of multi- 

 valve shells, as distinguished from the parietal valves, or those 

 which are arranged circularly and form the body of the shell. 

 OPERCULINA. D'Orb. A genus of microscopic Foraminifera. 

 OPERCULUM. (A cover or lid.) The plate or plates with which 

 many molluscous animals enclose the aperture of their shells, 

 when retired within them. The operculum is sometimes horny, 

 as in Trochus ; testaceous or shelly, as in Turbo. It is spiral 

 when from a central or sub-central nucleus, the successive layers 

 take a revolving direction, as in Trochus. It is concentric or 

 annular when the outside edge of each layer entirely surrounds 

 the preceding one. It is unguiculated, when the laminae are 

 placed side by side, as in Purpura. The opercula of multivalve 

 shells are composed of two or four pieces, which are called the 

 opercular valves. The shelly or membranaceous plate with which 

 some of the animals enclose the aperture of their shells, during 

 the wintry part of the year, for the purpose of protecting them 

 while in a torpid state, and which they get rid of by dissolving 

 the edges, when preparing to emerge from their temporary re- 

 tirement, must not be considered as the operculum, as it does 

 not belong to or form part of either the animal or its shell, but 

 is produced for the occasion by a secretion of the animal, being 

 deposited in a soft state and subsequently hardening. It is 

 called the epiphragm, and may easily be distinguished from the 

 true operculum by the texture, and by the circumstance of their 

 being soldered to the edge of the aperture. The operculum, 

 on the contrary, is moveable, and is always composed of a series 

 of successive layers, corresponding with the growth of the shell. 

 OPIS. Defr. A genus described by De Blainville as consisting 





