3. Genus _Pinna_. Pl. VIII. _Animal._ Body oval, elongated, moderately thick, enveloped in a mantle closed above, open below, and especially to the rear, where it forms sometimes a sort of tube furnished with tentacular cirri: a flabelliform abdominal appendage, and a very considerable byssus; mouth provided with double lips, beside two pairs of labial appendages; a single large retractile muscle apparent. _Shell._ Subcornate, fibrous, brittle, regular, equivalve, longitudinal, triangular, pointed anteriorly, in which direction is the summit, which is straight, wide, and frequently truncated posteriorly; hinge dorsal, longitudinal, linear, toothless; ligament occupying nearly all the dorsal edge of the shell; a single and very wide muscular impression posteriorly; a trace of the anterior in the summit. Inhabits the Mediterranean Sea. Fifteen species. Pinna rudis. P. seminuda. P. nobilis. P. marginata. P. pectinata. P. ingens. P. varicosa. P. flabellum. P. angustina. P. squamosa. P. muricata. P. saccata. P. dolabrata. P. vexillum. P. nigrina. FAMILY XVII. MALLEACEA. Five genera. 1. Genus _Crenatula_. Pl. VIII. _Animal._ Unknown. _Shell._ Irregular, much flattened, subrhomboidal, subequivalve, gaping posteriorly; summit anterior; hinge longitudinal, dorsal, toothless; ligament submultiple, or inflated from place to place, and inserted in a series of round cavities corresponding with the dorsal edge; muscular impression unique and subcentral. Inhabits the Red Sea. Seven species. Crenatula modiolaris. C. avicularis. C. nigrina. C. bicostalis. C. viridis. C. mytiloides. C. phasianoptera. 2. Genus _Perna_. Pl. VIII. _Animal._ Body much compressed, the mantle prolonged posteriorly in a sort of lobe, and fringed at its inferior edge only; a byssus; a single contractile muscle. _Shell._ Irregular, much compressed, subequivalve, form somewhat variable, gaping at the anterior part of its inferior edge; summit very small: hinge straight, vertical, buccal, toothless; ligament multiple, and inserted in a series of longitudinal and parallel furrows; a subcentral muscular impression. Inhabits the Indian seas. Ten species living. Two fossil; one found in France and one in Virginia. Perna ephippium. P. isognomon. P. femoralis. P. marsupium. P. obliqua. P. sulcata. P. vulsella. P. nuclea. P. avicularis. P. canina. 3. Genus _Malleus_. Pl. VIII. _Animal._ Imperfectly known, but certainly byssiferous, with a single retractile muscle.