Species in which the cardinal teeth are almost obsolete, in consequence of the enlargement of the pit of the ligament. _M. stultorum._ The fool’s Mactra. Pl. 9, fig. 6. Species in which all the teeth are very large, lamellous, striated longitudinally; colour reddish brown. _M. solida._ The solid Mactra. Species thick, solid, without epidermis; lateral teeth finely striated. _M. trigonella._ The three-cornered Mactra. Species in which the lateral teeth are almost obsolete; exterior surface smooth. _M. triangularis._ The triangular Mactra. Species very small, strong, opaque, white; inside white; margin strongly crenated. _M. crassa._ The thick Mactra. Species very thick, solid, striated longitudinally; the cardinal teeth obsolete, or almost so; the lateral very thick, very close, and reflected; an external ligament as well as an internal one. 3. Crassatella. Eleven species. May be easily known from the Mactra and Lutraria, as the valves, when closed, fit exactly, and do not gape. It is remarkable that all the living species contained in this genus only exist in the seas of Australasia, while at least seven species in a fossil state are found in France. Shell inequilateral, sub-orbicular, close, equivalve, sometimes attenuated at one end; two divergent primary teeth, with a cavity at the side; lateral teeth obsolete; ligament internal, inserted in the cavity of the hinge. Crassatella Kingicola. C. donacina. C. sulcata. C. subradiata. C. contraria. C. cuneata. C. rostrata. C. glabrata. C. erycinæa. C. cycladea. C. striata. _C. sulcata._ The furrowed Crassatella. Pl. 6, fig 4. Shell ordinarily thick, striated transversely, denticulated, sub-triangular, equivalve, inequilateral, summits well marked and evidently turned forward; hinge very large, subsimilar, formed by two diverging cardinal teeth, separated by a large pit; ligament almost entirely interior, and inserted in the pit. _C. Kingicola._ The King’s Island Crassatella. Ovate, orbicular; yellowish white, with obsolete rays; very minutely striated transversely; the umbones somewhat plicated. 4. Erycina. One species. The only living species of this genus is found on the sand in New-Holland, but there are many fossils in France. It is so equivocal in character that it is difficult to judge of their hinge. _E. cardioides._ The cardium-shaped Erycina. Pl. 6, fig. 5.