ORTHIS. 209 



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. pi. 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. (OrbtSj an orb.) Fam. Brachiopoda, Lam. 

 Palliobranchiata, Bl. — Descr. Inequi valve, 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. 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. 0. lsevis, 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 resem- 

 bling Solarium. 0. 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, 0. crassus, fig. 480, 0. discus. 



OREAS. Montf. Part of Cristellaria, Lam. A genus of micro- 

 scopic Foraminifera. 



ORTHIS. Dalman. (6pdog,orthos, straight.) Fam. Brachiopoda, 

 Lam. One of the generic divisions of Brachiopoda by Dalman, 

 thus described: "Hinge rectilinear, with umbones distant ; the 



