Tbe Pearly Fresh-water Mussels 



larger than outer pair ; all occupied by marsupia in breeding time, 

 forming smooth pads. Ninety-nine species. 



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. 



Q. 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. 



Q, 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. 



Q. ebena, Lea, is a soHd, 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. 



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. ^sopus, 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. 



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