OETHOCEEATA. 223 



Palliobrancliiata, Bl. — Bescr. Inequiralve, irregular, sub-orbi- 

 cular, compressed, attached by a fibrous substance passing 

 tbrougb a fissure near the centre of tbe lower valve ; upper valve 

 pateUiform, with the umbo central ; muscular impressions four 

 in each valve, semilunar. South America and "West Indies. — 

 Ohs. 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. Mr. Sowerby's Monograph of this genus in the 

 author's Thesaurus Conchyliorum contains 6 species. O. laevis, 

 PL xi. fig. 201. 



OEBICTJLAE,. (Orbiculus, a little orb.) Of a round or circular 

 form. 



OEBIC"ULI]S"A. Lam. A genus of microscopic Poraminifera. 



OEBIS. Lea. A minute fossil, described as " orbicular, with flat 

 quadrate whorls and aperture square," in other respects resem- 

 bling Solarium. O. Eotella, PI. xvi. fig. 355, 356. 



ORBITINA. Eisso. A genus said to be established upon the 

 nuclei of two land shells. 



OEBTJLITES. Lam. A genus separated from Ammonites on 

 account of the last volution covering the spire. This is generally 

 considered as characterizing the ISTautili, and distinguishing 

 them from the Ammonites ; but there are so many gradations 

 that it seems impossible to maintain the distinction in this 

 respect. Pig. 479, 0. crassus, fig. 480, O. discus. PL xxii. 



OREAS. Montf. Part of Ceistellaeia, Lam. A genus of 

 microscopic Poraminifera. 



OETHIS. Dalman. {dpdog, 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, PL xi, fig. 207. 



OETHOCEEA. Lam. See Nodosaeia. 



OETHOCEEATA. Lam. A famUy of Poly thalamous Cephalopoda, 

 Lam. containing the following genera: — 



