_Shell._ Close, suborbicular or transverse, striated longitudinally, denticulated, regular, equivalve, inequilateral, summits well marked, and turned to the front: primary teeth somewhat divergent with a hollow at the side; no lateral teeth nor obsolete ones; ligament internal and inserted into a pit in the hinge. It is easily known from the Mactra and Lutraria by the valves fitting exactly. The living species of this genus only exist in the seas of Australasia, while in a fossil state we find at least seven species in France. Eleven species. Crassatella kingicola. C. donacina. C. sulcata. C. rostrata. C. glabrata. C. subradiata. C. contraria. C. cuneata. C. Erycinæa. C. cycladea. C. striata. 4. Genus _Erycina_. Pl. VI. _Animal._ Unknown. _Shell._ Somewhat longer than high, subtrigonal, regular equivalved, inequilateral, gaping but little or not at all; summits well marked and a little inclined to the front; hinge subsimilar; two unequal cardinal teeth converging to the summit; two oblong, compressed, short, and inserted lateral teeth; ligament internal and situated in a pit. Owing to the equivocal character of this genus it is somewhat difficult to judge of the hinge. Inhabits the Indian and American seas. Two species. Erycina cardioides. E. striata. 5. Genus _Ungulina_. Pl. VI. _Animal._ Unknown. _Shell._ Vertical or sublongitudinal, a little irregular, not gaping, equivalve, subequilateral, with summits a little marked; hinge dorsal, formed by a cardinal tooth; two long muscular impressions, one short primary cleft tooth in each valve, with an oblong groove divided in the middle of the margin, ligament internal inserted in a pit. Two species. Ungulina oblonga. U. transversa. 6. Genus _Solemya_. Pl. VI. _Animal._ Unknown. _Shell._ Covered with a thick epidermis, enclosing it entirely except at the extremities, regular, somewhat thick, oval, elongated, straight and parallel edges, also rounded at its two extremities, valves equal, very inequilateral; the anterior side much longer than the posterior; summits feebly marked; hinge subsimilar, formed by a cardinal tooth, dilated, compressed, and a little reflexed above; subexterior ligament inserted upon the tooth, and nearly at the posterior extremity of the shell; two small, rounded, widely separated muscular impressions, without any visible abdominal impression. Four species. Solemya Australis. S. Mediterranea. · · · · · S. velum.* S. borealis.* 7. Genus _Amphidesma_. Pl. VI. _Animal._ Very imperfectly known.