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. 



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 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. 



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 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. 



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