3. _Solenacea_ (plates, fig. 60 to 68), longitudinally (transversely, Lam.) elongated, open at the anterior and posterior extremities. This family contains the genera Solen, Pholadomya, Panopæa, Glycimeris (Solecurtus) and Solenimya. 4. _Myaria_ (plates, fig. 69 to 76), ligament internal. A spoon-shaped ligamentary pit in one or both valves. Shell generally gaping at one or both extremities. This family includes the genera Anatina, Mya, Anatinella, Lyonsia, Myochama, Cleidotherus. 5. _Mactracea_ (plates, fig. 77 to 88), the cartilage placed in a trigonal pit, with a small external ligament. The genera Lutraria, Mactra, Crassatella, Erycina, Ungulina, Amphidesma, and Solenimya belong to this family, the last of which ought to have been placed among the Solenacea, as above. 6. _Corbulacea_ (plates, fig. 89, 90), inequivalve, with an internal ligament resembling the Mactracea, but differing in having one valve deeper than the other, although regular shells. This small family contains only the genera Corbula and Pandora. 7. _Lithophagidæ_ (plates, fig. 91 to 97), irregular, terebrating, living in holes of rocks. The genera are Saxicava, Petricola, and Venerirupis. 8. _Nymphacea_ (plates, fig. 98 to 110), ligament external, generally placed upon a prominent fulcrum, which passes from the inside to the outside of the hinge; valves generally gaping at the extremities. This family contains the genera Sanguinolaria, Psammobia, Psammotæa, Tellinides, Corbis, Lucina, Donax, Capsa, and Crassina. 9. _Conchacea_ (plates, fig. 111 to 121), regular, having several cardinal teeth and sometimes lateral teeth. The Conchacea constitute one of the most beautiful and numerous families of the class; they present equivalve shells, which are always regular, unattached, and in general closed, especially at the sides; they are always more or less inequilateral. They are divided into the _fluviatile_ and _marine Conchacea_, the first containing the genera Cyclas, Cyrena, and Galathæa, found in rivers; and the second, Cyprina, Cytherea, Venus, and Venericardia. 10. _Cardiacea_ (plates, fig. 122 to 130). This family, which resembles the last in some general characters, are also regular and equivalve, and are generally provided with radiating ribs, which are seldom seen in the Conchacea. The genera enumerated in this family are Cardium, Cardita, Cypricardia, Hiatella, and Isocardia. 11. _Arcacea_ (plates, fig. 131 to 138). These are known by having a row of numerous small teeth on the cardinal hinge in each valve. The genera included are, Cucullæa, Arca, Pectunculus, Nucula.