CERITHIUM. Brug. _Fam._ Canalifera, Lam. Entomostomata, Bl.--_Desc._ Elongated, ribbed, tuberculated, or rarely smooth, with a lengthened, turrited, pointed, pyramidal spire, consisting of numerous whorls; aperture sub-quadrate, terminated anteriorly by a tortuous canal; outer lip thickened, sometimes reflected, expanded; inner lip thickened posteriorly; operculum horny, spiral, with numerous whorls.--_Obs._ The fresh-water shells described as Cerithia by Lamarck, are separated under the name Potamis, and may be known by the thick, horny epidermis. Triphora, Desh. has the canal closed, except at the extremities. Cerithium Telescopium, does not appear to present the same characters as the other Cerithia, and has been separated by some writers under the generic name Telescopium. Cerithium Aluco, fig. 372. Mediterranean, East and West Indies, Coasts of the Pacific, Gallapagos, Australia, &c. Some small species are British. Fossils are numerous in the tertiary beds. CERVICOBRANCHIATA. Bl. The second order of Paracephalophora Hermaphrodita, Bl. containing symmetrical patelliform shells, divided into the families Retifera and Branchifera. CETOCIS. Montf. _Fam._ Orthocerata, Lam. and Bl. Placed by De Blainville in his section of Belemnites, characterized as having small folds at the apex. _Ex._ B. Penicillatus. CETOPIRUS. Ranz. CORONULA BALÆNARIS, Auct. fig. 16. CHAMA. Auct. _Fam._ Chamacea, Lam. and Bl.--_Descr._ Inequivalve, irregular, thick, foliaceous, attached by the umbo of the lower and larger valve. External ligament placed in a groove, following the curve of the umbones. Umbones spiral, coiled round on the back of the valves; hinge with a thick, crenated, lengthened tooth, in one valve, entering a corresponding cavity in the hinge margin of the other; muscular impressions, two in each valve, distinct, lateral.--_Obs._ The Linnæan genus Chama, included the beautiful shells now called Tridacna. These are exceedingly different from the true Chama, being regular and unattached. The Chama (Tridacna) gigas, when at its full age and development, is the largest shell known. Specimens have occurred weighing upwards of 500 lbs., and measuring two feet across. Diceras may be known from Chama by the spiral horns into which the umbones are produced; Isocardia, by the regularity of the shells, and it is hardly necessary to mention Spondylus, which may be known by the triangular disc between the umbones; Cleidothærus, Stutch. which resembles Chama in general form, has a separate bony appendage attached to the hinge, and may, moreover, be distinguished by its elongated muscular impression. Fig. 153, C. Lazarus. E. and W. Indies. CHAMACEA. Bl. The seventh family of the order Lamellibranchiata, Bl. containing the genera Chama, Diceras, Etheria, Tridacna, Isocardia and Trigonia.