The type described above is Q. metanevra, Raf. It occurs in the Mississippi drainage area except in its southern portion, and extends to the Tennessee and Arkansas rivers. It is one of the striking species. A row of nodules go down the posterior ridge.
undulata, Barnes, has wavy radiating ridges from the beaks, forming sharp points. It is the "blue point" of the pearl button factories.
Habitat.— St. Lawrence basin, Red River of the North, Lake Winnepeg, Mississippi basin, Alabama River system, wes tern and southwestern Texas.
plicata, Say, is a handsome, ribbed species closely re lated to undulata. It is a "blue point." Habitat.— Red River of the North, northward; Mississippi drainage to the Tennessee and Arkansas rivers.
ebena, Lea, is a solid, rounded or ovate shell with black epidermis. The high beaks curve inward and forward over a distinct lunule. There is a feeble posterior ridge. The pearl is thick and white. This is the "niggerhead" used as the standard of value in the button industry.
PLEUROBEMA, Agassiz Shell solid, triangular to rhomboid, with prominent umbonal region. Beaks at or near anterior end of shell, incurved and pointing forward over a small lunule; beaks sculptured with a few irregular upturning ridges; posterior ridge low, rounded; epidermis showing plainly the different periods of growth, tawny or olive, with squarish• spots forming rays; hinge strong, plate narrow, teeth triangular, ragged, laterals double; nacre silvery; shells alike. Animals yellow to salmon red, sometimes brown or blackish; marsupia occupying outer gills entire; ova sacs often paired. Seventy-two species. United States, chiefly in southern rivers.
Green, is the "bull-head" of the pearl button factory. The front part of the shell is very thick, the back part thin. The epidermis darkens with age.
375 The Pearly Fresh-water Mussels Habitat.— Ohio, Cumberland and Tennessee river systems, west to Missouri and Minnesota.
Genus UNIO, Retzius Shell oval to elongated, inequilateral, rounded in front, pointed or biangulate behind; with a posterior ridge, often arcuate when old; beaks not very full, sculptured with coarse ridges, doubly looped or broken ; surface of valves smooth or concentric ally ridged or pustulous; epidermis dull, sometimes faintly rayed; hinge plate narrow; teeth single in right valve, double opposite; cavity of beaks not deep nor compressed. Outer gills swollen into
smooth pads when filled with young; gills attached their whole length to the mantle behind. Species 145, inhabiting all the northern hemisphere above the Tropic of Cancer, except the Pacific slope and Southeastern Asia.
Linn., has its name from an ancient use to which the single valves were formerly put. They were used to hold artists' colours. Very common and easily obtained, shallow but stable, pearly lined — they served the painter's purpose exactly. Doubtless many artists keep to old traditions, scorning the newer porcelain utensils in the modern artist's "kit." The oblong, compressed valves with the low, eroded beaks well for ward, have a thin epidermis, and concentric brown lines, the pos terior area only rayed with green. The animal is red,with a broad, tongue-shaped foot used in burrowing into the mud. The mantle border is brownish, and united to form the two siphonal orifices.
The wide range of this species and the great amount of at tention paid to its forms by conchologists of high and low degree account for the long list of synonyms in Mr. Simpson's report. Length, about 3 inches.
Habitat.— Europe and eastward, at least to the Lena River.
is the best known American species. It is elongately trapezoidal, scarcely inflated, nearly straight on the ventral margin, with small beaks depressed, well forward, and sculptured with a few coarse parallel lines. The shining epi dermis is faintly rayed, but becomes roughened and the rays obsolete with age. Length, 3 to 4 inches.
Habitat.— River systems of the Atlantic region from St. Lawrence to Georgia, west in Canada to Manitoba.
376 The Pearly Fresh-water Mussels U. spinosus, Lea, is an isolated species, distinguished by sharp thorn-like spines, one or two on each valve, near the beak. It is found in the Altamaha River, Georgia.