series of successive layers, corresponding with the growth of the shell. OPIS. Defr. A genus described by De Blainville as consisting of species of Trigonia which have the umbones sub-spiral, with a large, striated tooth on the hinge. Opis cardissoides, Trigonia, Lam. Opis similis, Sow. Min. Con. pl. 232. f. 2. ORAL. (_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. (_Orbis_, an orb.) _Fam._ Brachiopoda, Lam. Palliobranchiata, Bl.--_Descr._ Inequivalve, irregular, sub-orbicular, compressed, attached by a fibrous substance passing through a fissure near the centre of the lower valve; upper valve patelliform, with the umbo central; muscular impressions four in each valve, semilunar. South America and West Indies.--_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. lævis, fig. 201. ORBICULAR. (_Orbiculus_, a little orb.) Of a round or circular form. ORBICULINA. Lam. A genus of microscopic Foraminifera. ORBIS. Lea. A minute fossil, described as "orbicular, with flat quadrate whorls and aperture square," in other respects resembling Solarium. O. Rotella, fig. 355, 356. ORBITINA. Risso. A genus said to be established upon the nuclei of two land shells. ORBULITES. Lam. A genus separated from Ammonites on account of the last volution covering the spire. This is generally considered as characterizing the Nautili, and distinguishing them from the Ammonites; but there are so many gradations that it seems impossible to maintain the distinction in this respect. Fig. 479, O. crassus, fig. 480, O. discus. OREAS. Montf. Part of CRISTELLARIA, Lam. A genus of microscopic Foraminifera. ORTHIS. Dalman. ([Greek: orthos], _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 triangular pit." O. basalis, fig. 207. ORTHOCERA. Lam. See NODOSARIA. ORTHOCERATA. Lam. A family of Polythalamous Cephalopoda, Lam. containing the following genera:-- 1. CONULARIA. Conical, externally striated; no siphon. Fig. 469. 2. AMPLEXUS. Cylindrical; margins of the septa reflected. Fig. 463. 3. ORTHOCERATITES. Straight, gradually conical; septa simple; siphon central. Fig. 464. 4. NODOSARIA. Divided externally into lobes. Fig. 465. 5. BELEMNITES. Straight, conical; septa simple; siphon lateral; apex solid; internal cast, or nucleus, pyramidal, separable. Fig. 466 to 468.