CORRODED. (_Corrodo_, eat away, consume.) The umbones, apices, and other thick parts of shells, are frequently worn away or consumed by the action of the element in which they exist. As the thickest parts of some shells are the most subject to this operation, it appears to the author to arise from the outer surface of the shell, being less under the influence of the animal juices than the other parts; and therefore, more exposed to the influence of the surrounding element. This, however, is not the case with respect to the Nayades and other fresh-water shells; with these, corrosion does not take place until after the thick epidermis which covers them, becomes wounded by some means or other, and then the animal thickens its shell within as fast as it is corroded without. CORTALUS. Montf. (Conch. Syst. 1. 115.) A genus of microscopic Foraminifera, placed by De Blainville in a division of the genus Rotalites. COSTATED. Ribbed, as Cardium Angulatum, fig. 123. COSTELLARIA. A sub-genus of the genus Tiara, Sw. (Mitra.) C. rigida. Swainson, Zool. Ill. 1st series, pl. 29. COWRY. A common name for shells of the genus Cypræa. CRANIA. (_Cranium_, a skull.) _Fam._ Rudistes, Lam. _Order_, Pallio-branchiata, Bl.--_Descr._ Inequivalve, equilateral, irregular, sub-quadrate; upper valve patelliform, conical, with the umbo near the centre; lower valve attached by its outer surface; muscular impressions, 4 in each valve; two large, posterior, distant; two small, near to each other, central. No hinge teeth; no ligament.--_Obs._ This genus properly belongs to the Brachiopoda, Lam. It differs from Orbicula in the mode of attachment, which in the latter, is by a byssus passing through the lower valve, and not by the valve itself. Hipponyx has only two muscular impressions in each valve. The name of this genus is derived from the inner surface of the attached valve, which presents a remarkable resemblance to the facial portion of a human skull. This appearance is caused by the situation and elevated edges of the muscular impressions. Fig. 197. Coasts of Britain and Mediterranean. CRASSATED. (_Crassus_, thick.) Used to express a thickness in the substance of a shell. _Ex._ Glycimeris, fig. 67.