Pearly Fresh-water Mussels — Part 1

Genus 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.

P. AEsopus, 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.

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 concentrically 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.

U. pictorum, 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 procelain utensils in the modern artist's "kit." The oblong, compressed valves with the low, eroded beaks well forward, have a thin epidermis, and concentric brown lines, the posterior 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 attention 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.

U. complanatus, Dillw., 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 epidermis 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.

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.

U. gibbosus, Barnes, is a mid-continental Unio of wide distribution. The shell is solid, triangularly or elongately ovate, arcuate when old, gibbous above, rounded in front, pointed or biangulate behind, with a moderate posterior ridge; beaks low, sculptured with a few strong, parallel ridges; epidermis cloth-like, dull; hinge teeth strong, rough; laterals club-shaped, vertically striated or granular; one or more furrows and ridges run parallel to laterals; muscle scars distinct, deep; mantle thin, with thickened edges; gills large, curved at ventral border. Several varieties are recognised. Length, 3 to 5 inches.

Habitat.-Entire drainage areas of St. Lawrence, Mississippi and Alabama Rivers; also to Guadaloupe River, Texas, and into Florida.

Genus ANODONTA, Lam.

Shell thin, elliptical, often slightly winged posteriorly, inflated; beaks with fine parallel ridges, sometimes looped and nodulous; hinge toothless, reduced to a curved line; surface smooth, shining; pearl dull. Sexes with shells alike; marsupia filling whole of outer gills.

The Swan Mussel (A. cygnea, Linn.) is spread over Europe and Siberia. Its synonyms are legion, owing to its variability, and the tendency of "new school" conchologists to give slight variations the rank of specific characters.

A. grandis, Say, is a large and widely distributed American species. It inhabits the entire Mississippi system, the St. Lawrence and Red River of the North, Lake Winnipeg, Manitoba, extending to Southeastern Pennsylvania and into Texas.

A. marginata, Say, is a fragile, elongated Anodonta, with green epidermis, and bluish white lining, found in the St. Lawrence drainage.

Genus LAMPSILIS, Raf.

Shell oval or elliptical, usually smooth, without posterior ridge; epidermis shiny, often brilliantly rayed; two hinge teeth with single lateral in right valve, two in left; female shell swollen in posterior ventral region to accommodate marsupia, which occupy the hinder part of outer gills; mantle margins doubled and thickened.