The family _Ætheriidæ_, or freshwater Oysters, consists of but three genera: _Ætheria_ contains African, and _Mülleria_ and _Barttettia_ Indian and South-American forms. When young the shells of _Ætheria_ (which are common in the Nile) are free and not unlike an _Anodonta_, but when adult they become attached and irregular and look like an olive-green Oyster; they are, however, provided with two muscular impressions instead of one, as in ordinary marine Oysters. Still more remarkable is _Mülleria lobata_ of Colombia, which, when young, freely moves about and has two adductor muscles, but in time becomes attached and stationary, and then possesses but a single adductor. [Sidenote: Cases 179–181.] The _Tellinas_ have usually thin shells, and their two siphons are longer and more completely separated from each other than in the _Veneridæ_. The pallial line is widely and deeply sinuated, and the ligament generally external. In the genus _Semele_ of the family _Scrobiculariidæ_, it is placed within the hinge-margin. They live in great numbers beneath the sand in shallow water, and are occasionally used as food. [Sidenote: Cases 183–185.] The _Mactridæ_ have an internal ligament to the hinge, the siphons are joined together and fringed at the ends, and the pallial line is more or less sinuated. _Spisula solidissima_ (Case 184), the largest species found on the coast of the United States, is a common article of diet. [Illustration: Fig. 32. Common British Cockle (_Cardium edule_). _a_, foot; _b_, exhalant siphon; _c_, branchial or inhalant siphon; _d_, edge of mantle; _e_, ligament; _f_, umbones or beaks of the shell. ] The next family, _Veneridæ_, have long respiratory siphons and a sinuated pallial line. Many of this tribe are very beautiful in form and colouring, and most of them have very hard strong shells. The valves are united above by an external ligament, and the hinge-plate is toothed. Nearly all of them live buried an inch or two beneath the sand or mud, but a few are found burrowing in rocks. Probably the majority of the species of this family might be used as food. _Venus verrucosa_, of our own southern shores, is frequently eaten both in this country and abroad; and _Venus mercenaria_ (Case 189) is commonly sold in the markets of Philadelphia and New York. _Cytherea lusoria_ (Case 185) also forms a favourite article of diet among the poorer classes in Japan, and several kinds are eaten by the natives of New Zealand and other countries. [Sidenote: Cases 192–194.] Some of the “Cockles” (_Cardiidæ_) from warm latitudes are highly coloured and adorned with most beautiful sculpture. Probably the majority are eatable, as the common cockle (_Cardium edule_) of the British coast. The foot of these molluscs is very large, bent, and used for leaping. The siphons are short and fringed at the margins. [Sidenote: Cases 195–196.]