Shell rather longer than high, sub-triangular, regular, equivalve, inequilateral, rarely gaping; the summits well marked and a little anteriorly inclined; hinge subsimilar; two unequal cardinal teeth, converging at the summit, and leaving a pit between them; two lateral teeth, not distant, lamellous, inserted; ligament interior, fixed in the cavity between the primary teeth. 5. Ungulina. Two species. This genus is very remarkable for having the pit or cavity divided into two parts, the one at the end of the other; the ligament is partially seen from the outside. Ungulina oblonga. Ungulina transversa. _U. transversa._ The transverse Ungulina. Shell vertical or longitudinal, rather irregular, not gaping, equivalve, inequilateral, with summits little marked and decorticated; hinge dorsal, formed by one short, primary cleft tooth, before an oblong pit, divided by a small ligament, in which is inserted a sub-interior ligament; colour yellowish brown. 6. Solenimya. Two species. This genus, which at first sight is confounded with the Solens, differs from them particularly by the singular disposition of the ligament placed at the short side of the shell. Solenimya Australis. Solenimya Mediterranea. _S. Australis._ The Australian Solenimya. Pl. 6, fig. 2. Shell covered with a thick brownish epidermis, regular, thick, elongated oval, edges straight and parallel, equally rounded at both extremities; valves equal, very inequilateral. _S. Mediterranea._ The Mediterranean Solenimya. Transversely oblong; dark brown, ribbed longitudinally, with imbricated projecting foliations; inside white. 7. Amphidesma. Sixteen species. This genus was constituted by Lamarck on account of the peculiar characters which distinguished it from those genera which it most resembles; particularly in having the valves connected by two ligaments. Shell generally small, transverse, sub-oval or rounded, occasionally a little gaping at the sides; hinge with one or two cardinal teeth, and a narrow cavity for the interior ligament; exterior ligament short. Amphidesma variegata. A. donacilla. A. lactea. A. cornea. A. albella. A. flexuosa. A. prismatica. A. phaseolina. A. corbuloides. A. glabrella. A. lucinalis. A. Boysii. A. tenuis. A. purpurascens. A. nucleola. A. physioides. _A. glabrella._ The smooth Amphidesma. Pl. 6, fig. 9. Species lenticular or oval, with or without a lunated depression.