NOTE.—_Alasmidonta._ Separated by Say from the Anodonta by its primary tooth, and from the Unio by being destitute of the lamelliform teeth. _Shell._ Transverse, equivalve, inequilateral; hinge with a primary tooth in each valve; cicatrices three. Four species. Alasmidonta undulata. A. ambigua. A. marginata. A. confragosa. 3. Genus _Anodonta_. Pl. VIII. _Animal._ As above. _Shell._ Rather thin, regular, close, equivalve, inequilateral; summit anterodorsal; hinge entirely without teeth, with a postapicial lamina; ligament external, dorsal, and postapicial, two well marked muscular impressions, besides those of the retractile muscles. Inhabits fresh-water lakes in Europe and America. Forty-nine living species; and many fossil. Anodonta areolatus. A. marginata. A. rubens. A. anatina. A. fragilis. A. trapezialis. A. rufa. A. uniopsis. A. intermedia. A. glauca. A. anatina. A. sinuosa. A. Patagonica. A. cygnæa. A. sulcata. A. cataracta. A. exotica. A. crispata. A. Pennsylvanica. A. maximus. The following species are American. Anodonta magnifica. _Lea._ A. Woodiana. „ A. angulata. „ A. Benedictensis. „ A. cylindracea. „ A. decora. „ A. edentula. „ Anodonta gigantea. _Lea._ A. incerta. „ A. Pepiniana. „ A. plana. „ A. salmonia. „ A. Stewartiana. „ A. subcylindracea. „ A. Ferussacina. _Lea._ A. fluviatilis. „ A. Newtonensis. „ A. Nuttalliana. „ A. Oregonensis. „ A. ovata. „ A. pavonia. „ A. Wahlamatensis. „ A. Wardiana. „ A. gibbosa. _Say._ A. grandis. „ A. impura. „ A. suborbiculata. „ A. undulata. „ A. lugubris. „ 4. Genus _Iridina_. Pl. VIII. _Animal._ As above. The principal difference between the Anodonta and the Iridina, consists in the tuberculated hinge of the latter. _Shell._ Thin, oval, much elongated, inauriculated; hinge very long, linear, and crenulated throughout its length, ligament external and marginal; two distinct muscular impressions. Inhabits the Nile and American coasts. Six species. Iridina exotica. I. Nilotica. · · · · · I. cælistis.* I. rubens.* I. elongata.* I. McMurtria.* FAMILY XIV. CHAMACEA. Three genera. 1. Genus _Diceras_. Pl. VIII. _Animal._ _Shell._ Inequivalve, adherent; beaks conical, very large, diverging in irregular spiral contortions; hinge with a large, thick, concave, subauricular tooth in the larger prominent valve; two muscular impressions. One species. Fossil. Diceras arietina. 2. Genus _Chama_. Pl. VIII. _Animal._ Body suborbicular terminated superiorly by a sort of hook; mantle very slightly opened for the passage of a foot, terminated at its extremity by a part much narrower than the base; superior lobes of the branchiæ very short.