Easily distinguished from the Unio by a compound cardinal tooth, which slopes in an inclined position towards the posterior side. They are more found in lakes than in rivers. Shell solid, pearly, equivalve, obliquely triangular, auriculated; base truncated and straight; hinge with two projecting teeth; the cardinal divided into numerous divergent parts; anterior ones smaller, the others long and lamellar. Hyria avicularis. Hyria corrugata. _H. avicularis._ The little bird Hyria. Pl. 5, fig. 4. Umbones smooth and polished; ears large, with pointed terminations; finely striated; interior pearly, exterior of a rich reddish golden-yellow colour; covered with a greenish brown epidermis. 3. Anodonta. Fifteen species. A fresh-water shell, found in ponds and lakes, difficult to be distinguished from the Unio but by the hinge, which wants the cardinal and lateral tooth, and merely presents a smooth internal rim round the edge terminated by a sinus or notch, in which the anterior extremity of the ligament is sunk; the substance is pearly, covered with a false epidermis. Shell ordinarily rather thin, regular, close, equivalve, inequilateral; summit anterodorsal; hinge linear, without teeth; ligament external, dorsal, and post-apicial; two well-marked muscular impressions. Anodonta cygnæa. A. anatina. A. sulcata. A. fragilis. A. cataracta. A. trapezialis. A. exotica. A. rubens. A. crispata. A. uniopsis. A. Pennsylvanica. A. intermedia. A. glauca. A. sinuosa. A. Patagonica. _A. cygnæa._ The Swan Anodonta. Pl. 11, fig. 2. Species oval, thin, elongated, hinge straight, and only auriculated anteriorly; beaks small and ventricose; concentrically wrinkled; covered with a greenish epidermis, which is frequently of a brown tinge towards the umbo. _A. rubens._ The ruddy Anodonta. Species oval, hinge arched, without trace of auricula or ear. _A. trapezialis._ The trapezium Anodonta. Pl. 11, fig. 3. Species oval or rounded, auriculated on both sides the summit. 4. Iridina. One species. A very rare shell, separated from the Anodonta on account of the hinge being attenuated in the middle, and having small tubercles distributed along its length. The substance is thicker and more solid than the Anodonta, and of a brilliant rose-coloured pearly hue; it is found in the rivers of warm countries.