"_Potamophila_ is a scarce river-shell from Ceylon. Some species have also been brought from Congo by African travellers. The form is triangular, very thick, covered with an olive-green epidermis. Lamarck's two genera have been still further reduced; but I shall refer you to the Museum for their new names. Observe, in _Venus_ there are three cardinal teeth, close together, on each valve, with divergent lateral teeth. _V. lamellata_ is rare and beautiful, from the seas of New Holland. There are many species of _Venus_ in a fossil state. In _Cytherea_ we find four cardinal teeth on the _right_ valve, three of them near together, the fourth quite apart. The _left_ valve has three cardinal teeth. _C. Dionè_, the thorny Venus, is a pretty shell with spines. You may easily procure it. "_Astarte_ has some fossil species in the crag and green sand: _A. obliquata_ is one species. "_Venericardia_ is wholly a fossil genus: one species is found in the crag, _V. senilis_. * * * * * "The family _Cardiacea_ contains _Cardium_ _Cockle_ _Cardita_ _Chama_ (some species.) _Cypricardia_ ------ _Hiatella_ _Mya_. _Isocardia_ _Chama_. "_Isocardia cor_ is British. (Plate 9.) There is a beautiful species, _Isocardia moltkiana_ from the East Indies, which is much valued by collectors. * * * * * "In the family _Arcacea_ we find, _Cucullea_ _Arca_. _Arca_ _Arca_, ark-shell. _Pectunculus_ _Arca_. _Nucula_ _Arca_. [Sidenote: ARCA. PECTUNCULUS. NUCULA. NAYADA.] "The hinge of _Arca_ in this arrangement is always _straight_, furnished with a number of teeth; the ligament is external. The shells are open at one end, for the animal throws out at the aperture a number of threads, by which it fastens itself to the rocks. The species are thirty-seven, and also several fossil. "The orbicular form of _Pectunculus_, and its arched hinge, distinguish this genus from the preceding one. They are allied to the Pectens by their form, and their crenulated internal margin. "The hinge of _Nucula_ is set with little teeth on each side, like a comb. It is pearly within, and sometimes small pearls are found in the shell. _Pectunculus costatus_ is found in London clay. "_Trigoniana_ is a small family containing _Trigonia_ and _Castalia_. The first is a fossil genus chiefly. Some species are found in the Portland stone, or oölite beds. "The next family contains the _Nayada_, chiefly composed of fluviatic, or fresh-water shells. They are covered with an olive-brown epidermis, which is constantly found eroded, or destroyed at the beaks. The muscular impressions are lateral and much separated; one of them is formed of two or three distinct irregular impressions.