_A. lactea._ The milky Amphidesma. Sub-orbicular, sub-pellucid, compressed, reticulated; yellowish white. FAMILY VI. CORBULACEA. Two genera. 1. Corbula. Nine species. This genus approximates the Crassatella and Ungulina, but is distinguished from them by the inequality of the valves and the strong primary tooth. Shell rather solid, a little irregular and triangular, inequivalve, more or less inequilateral, rounded and enlarged before, attenuated and prolonged behind; summits well marked, one projecting behind the other; hinge anomalous, formed by a large, conical, recurved cardinal tooth, with a cavity at its base for the reception of the tooth of the other valve; ligament very small; two muscular impressions little distant. Corbula Australis. C. sulcata. C. erythrodon. C. ovalina. C. Taitensis. C. nucleus. C. impressa. C. porcina. C. semen. _C. nucleus._ The kernel Corbula. Strong, sub-triangular, under valve larger than the upper one; transversely striated; covered with a thick brownish epidermis. _C. ovalina._ The ovate Corbula. Pl. 6, fig. 6. Regular species. _C. Australis._ The Australian Corbula. Irregular species, living in stone. 2. Pandora. Two species. Closely allied to the Corbula. Pandora rostrata. Pandora obtusa. _P. rostrata._ The beaked Pandora. Pl. 6, fig. 3. Shell white, regular, elongated, inequivalve, inequilateral; right or upper valve entirely flat, with a plait or fold; much produced towards the beak; hinge anomalous, formed by a transverse cardinal tooth on the right valve, entering into a corresponding cavity on the left; ligament internal, oblique, triangular, inserted in a pit rather deep, with edges a little projecting on each valve; two rounded muscular impressions. FAMILY VII. LITHOPHAGI. Three genera. 1. Saxicava. Five species. This genus is taken from the Mytilus, and, like the Pholas, possesses the faculty of penetrating calcareous rocks, from which it cannot be extracted without breaking the substance in which it is imbedded. Shell bivalve, thick, covered with epidermis, rather irregular, elongated, sub-cylindrical, obtuse at the two extremities; summits little marked; hinge toothless, or with a very small rudimentary tooth; ligament external, a little inflated. Saxicava rugosa. S. Gallicana. S. pholadis. S. Australis. S. veneriformis.