74 ORTHOCERATA. 
OPER'CULUM. (A cover, a 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 lamine are 
placed side by side,as in Purpura. The opercula of multivalve 
shells are composed of two or four pieces. 
ORBI'CULA. Lam. (Orbis, an orb.) Fam. Brachiopoda, Lam. 
Palliobranchiata, Bl. Deser. Inequivalve, irregular, sub-orbicular 
compressed, attached by a fibrous substance passing through a 
fissure near the centre of the lower valve; upper valve patelli- 
form, with the umbo central; muscular impressions four in each 
valve, semilunar. Obs. Discina, Lam. is an Orbicula. Crania 
is known from this genus by having no fissure in the lower valve, 
but being attached by its substance. Hipponyx has only two 
muscular impressions in each valve. O. levis, fig. 201. 
ORBI'CULAR. (orbiculus, a little orb.) Of a round or circular 
form. 
ORBICULI'NA. Lam. A genus of microscopic Foraminifera. 
OR’BIS. Lea. <A minute fossil, described as “orbicular, with flat 
quadrate whorls and aperture square,’’ in other respects resem- 
bling Solarium. O. rotella, fig.{355, 356. 
ORBITINA. Risso. A genus said to be established upon the 
nuclei of two land shells. 
OR’BULITES. Lam. A genus separated from Ammonites on 
account of the last volution covering the spire. This is gene- 
rally considered as characterizing the Nautili,and distinguishing 
them from the Ammonites; but there are so many gradations, 
that it seems impossible tomaintain the distinction in this respect. 
Fig. 479, O. crassa; fig. 480, O. discus. 
ORAL. (Os, oris, mouth.) Applied tothat part of a shell which 
corresponds with the mouth of the animal, but very seldom used 
in this sense. 
O’RCAS. Montf. Part of Caistetuaria, Lam. A genus of mi- 
croscopic Foraminifera. 
OR'THIS. Dalman. (ép6os, orthos, straight.) Fam. Brachiopoda, 
Lam. one of the generic divisions of Brachiopoda by Dalman, 
thus described : “‘ Hinge rectilinear, with umbones distant; the 
larger valve with a transverse, basal, smooth area, with a trian- 
gular pit.”’ O. basalis, fig. 207. 
ORTHOCERA. Lam. vide Noposarta. 
ORTHOCE/RATA. Lam. A family of Polythalamous Cephalo- 
poda, Lam. containing the genera BELEMNITES, (including 
Hibolithes,) Orthocera, Nodosaria, Hippurites, and Conilites. 
