168 LACUNA. 



sembling the Anodontje, Auct. but its peculiar characteristic 

 is that the hinge lamina is tuberculated or crenulaated in its 

 whole length. Sowerby unites all the genera of the family into 

 the genus Unio. Fig. 150, I. Elongata. 



IRREGULAR SHELLS, are those which, being attached to, or 

 imbedded in other marine bodies, have no constant form, but are 

 modified in shape according to the substances to which they are 

 fixed, as the Chamacea, fig. 153 to 155. 



IRUS. Oken. Comprehending Pandora, Petricola, Saxicava, 

 &c. 



ISOCARDIA. Lam. (Io-oe, isos, similar ; KapSia, cardia, heart.) 

 Fam. Cardiacea, Lam. Chamacea, Bl. — Descr. Cordiform, regular, 

 equivalve, ventricose, with distant, diverging, involute, free um- 

 bones ; hinge with two compressed cardinal, and one distant, 

 compressed lateral teeth in each valve ; ligament external, bifid, 

 diverging in the direction of the umbones. — Obs. The shells com- 

 posing this genus are remarkable for the beautiful curvature of 

 the diverging umbones. European and Chinese Seas. Fig. 126, 

 I. Moltkiana. 



KEEL. A flattened ridge, resembling the keel of a ship. As that 

 on the back of Carinaria vitrea, fig. 488, and those on the whorls 

 of some spiral shells. A shell characterized by a keel or keels is 

 said to be carinated. 



KELLIA. Turton. Mya Suborbicularis, Montague. 



LABIS. Oken. Monodonta, Lam. 



LABIUM, or inner lip, — -is used to express that side of the 

 aperture wliich is nearest to the axis and generally contiguous to 

 the body whorl. The lower part of this, when sufficiently dis- 

 tinct from that part which overwraps the body whorl, is called the 

 Columella. 



LAB RUM, or outer lip, — is the edge of the aperture at the greatest 

 distance from the axis. 



LA.CINEA. Humph. Chama, Lam. 



LACUNA. Turt. Fam. Turbinacea, Lam. — Descr. Globose, 

 thin, covered with a smooth epidermis ; spire short, consisting of 

 few rapidly increasing whorls ; aperture semilunar, rounded at the 



