Freshwater mussel breathing and eating.
Unio pictorum.—More oblong and thinner shell, yellowish, girdled with brown in the lines of growth. Common. The specific name recalls that gold and silver paint used to be sold in these shells (or marine mussels) for illuminating work. It is said to produce 220,000 eggs in the three summer months.
Unio (margaritana) margaritifer.—Shell solid and black, beaks always eroded. Mainly found in mountain streams. Its pearls are few and poor compared with those of marine shells; but they attracted the notice of Cæsar and so hastened the conquest (and development) of Britain.
Anodonta cygnea.—(In this genus the hinge is toothless, whence its generic name. The specific names cygnea and anatina mean “swan” and “duck,” in reference to their comparative size). This is the largest of our freshwater shells, reaching even 9 inches in breadth by 4-1/2 in length. Common in ponds and slow water. Sometimes the shells are yellowish green with rays of the same colour.
Anodonta anatina.—Doubtful if this is a separate species or only a smaller form. The hinge line is raised instead of being straight, and the posterior side slopes abruptly instead of gradually.
In the next family are two genera, Sphærium and Pisidium.
Sphærium corneum.—Very common. Shell somewhat globular, glossy, opaque, horn-coloured, marked with lighter bands in the line of growth. Usually on the bottom, but can suspend itself by threads of mucus.
Sphærium rivicola.—Much larger. Also flatter and more striated. Yellowish brown or greenish. A whole series of young of different sizes will be found in the animal.
Sphærium pallidum.—Local in canals and ponds. Oblong. Distinguished also from the previous species by the body being milk-white, and the shell is ashy-grey.