The most common variety of unio in American rivers, especially in the Mississippi river, is that known as the nigger-head (Quadrula ebena). It is also the principal species used for button-making. Similar is the warty-back (Quadrula pustulosa) so called because the shell has a number of warts or excrescences on the outside of the valves. The "bull-head" (Pleurobena Aesopus) is found in abundance with the nigger-head. It has a blackish-brown exterior, presenting several radiating ridges, and a white lining. The two latter are inferior as material for buttons as the shells are brittle. The mucket (Lampsilis ligamentinus) is a large shell, average size 4 inches, has a dark brown exterior and cream-white lining. It is too thin and brittle to make first class material for buttons though fine pearls are sometimes found in them. The sand-shells furnish good material for buttons. They are long, sometimes six inches, and narrow. They are usually found on sandy bottoms and are said to move from the channel toward the shores in the morning and back in the evening. The most abundant is the yellow sand-shell (Lampsilis anodontoides) so called from its bright yellowish brown exterior. Another kind, the black sand-shell (Lampsilis rectus) has a black epidermis. A smaller variety, less abundant now than formerly, is the slough sand-shell (Lampsilis fallaciosus). These are generally found in coves or the mouths of rivulets. The deer-horn or buckhorn (Tritigonia verrucosa) is a large variety, sometimes attaining a length of nine inches in the Iowa river, though the average in the Mississippi is about five inches. The shell, as the name indicates, has a rough, warty exterior. The supply is small and uncertain. Another rare species is the butterfly (Plagiola securis). It is a small, flat, thick shell of fine color, and the valves are butterfly in shape with a reddish-brown epidermis striped by darker radiating lines. It is abundant only in the Illinois and Ohio rivers. The hatchet-back, hackle-back, or heel-splitter (Symphynota complanata), is a large black mussel having a thin sharp-edged shell, one valve-edge projecting. It yields few pearls though fine specimens are occasionally found in this variety. The blue-point (Quadrula undulata) has a large, thick shell, with ridges on the exterior, curving round the umbones and extending to the edge. Like the black-edge meleagrina, the nacre at the edge is discolored. In this case by a bluish or purplish tint. Some idea of the enormous quantities of mussels contained in some of these beds in our western rivers may be gained from the reports of the fisheries in the first years of their discovery. Ten thousand tons of shells were taken in three years near New Boston, Ill., from one bed. Reckoned by the usual average this would mean not less than 100,000,000 shells. In some beds, the mussels have been found several feet deep, the bottom layers being dead.