222 OEBICULA. 



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 moUuscous animals enclose the aperture of their shells, 

 when retired withiu them. The operculum is sometimes horny ^ 

 as in Trochus ; and sometimes testaceous or sheUy, as in Turbo. 

 It is spiral when from a central or sub-central nucleus, the succes- 

 sive layers take a revolving direction, as in Trochus. It is con- 

 centric 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 BlainviUe as consisting 

 of species of Trigonia which have the umbones sub-spiral, vdth a 

 large, striated tooth on the hinge. Opis cardissoides, Trigonia, 

 Lam. Opis simiLis, Sow. Min. Con. pi. 232. f. 2. 



OEAL. (Os, oris, mouth.) Applied to that part of a shell which 

 corresponds with the mouth of the animal, but very seldom used 

 in this sense. 



ORBICULA. Lam. {Orbisy an orb.) Fam. Brachipoda, Lam. 



