Nucula lanceolata. N. rostrata. N. pella. N. Nicobarica. N. obliqua. N. margaritacea. _N. rostrata._ The beaked Nucula. Species of which the margin is entire. _N. margaritacea._ The pearly Nucula. Pl. 11, fig. 7. Species of which the margin is crenated; numerous regular pectinated teeth; obliquely ovate, trigonal; striæ minute and almost obsolete; covered with a greenish epidermis; inside silvery, pearl-like. FAMILY XII. TRIGONACEA. Two genera. 1. Trigonia. One species. Supposed to be in very deep places in the sea; it is a strong, beautiful, pearly shell, sub-trigonal or sub-orbicular; thick, regular, equivalve, inequilateral; summits slightly prominent, recurved, anterodorsal; hinge complex, dorsal, dissimilar; two large oblong teeth laterally compressed, joined angularly under the summit, strongly furrowed on the right valve, penetrating into two excavations of the same form, equally furrowed on the left valve; ligament exterior and marginal; two distinct muscular impressions. _T. pectinata._ The pectinated Trigonia. Pl. 11, fig. 4. Species sub-orbicular, with radiated or divergent, prominent, and somewhat scaly ribs; inside pearly; margin crenellated. 2. Castalia. One species. This genus is found in fresh waters, and differs from the Trigonia in the number and position of the lamellar teeth. The substance of the shell is pearly. Shell sub-trigonal, equivalve, inequilateral; umbones eroded, covered with epidermis, and inflected anteriorly; hinge with two lamellar teeth, transversely striated, one of them posterior, distant, and shortened, the other anterior, lengthened, and lateral; ligament exterior. _C. ambigua._ The ambiguous Castalia. Pl. 11, fig. 5. Short, sub-trigonal; umbones truncated; longitudinally ribbed, with distant transverse striæ; covered with epidermis, under which the shell is of a pale chestnut brown, inside pearly; the lamellar and præ-apicial teeth are well marked, more regular, and all striated perpendicular to their length. FAMILY XIII. NAIADES. Four genera. 1. Unio. Forty-eight species. The species of this genus become every day more numerous; they are found in all countries, but particularly in North America. The Unio is a fresh-water shell, and therefore, with great propriety, removed from the Mya, which consists entirely of marine shells. The substance is pearly; the exterior covered with a brown or green epidermis; the apices eroded. They are found in the mud of rivers, with their apices downward; some are slightly gaping, and some species produce fine pearls.