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with two large knotty ribs, oblique, with inter- 

 mediary depression ; edges flexuous / epidermis 

 reddish brown, almost smooth ; nacre white. 

 Breadth f diam. 4 axis f of the length. 



Var. 1. olivacea epidermis olive co- 

 loured, with flexuous wrinkles. 

 Var. 2. radiata with radiated pale 



lines. 



Var. 3. difformis with irregularly form- 

 ed hunches (bosselures) and depres- 

 sions. 



Very common in the Ohio and adjacent rivers ; 

 length from one to three inches ; lamellar tooth 

 short, oblique, obtuse ; fossule visible ; cardinal 

 tooth wrinkled ; apices prominent, heart-shaped. 

 This species strongly resembles the genus Obli- 

 quaria, but the cardinal tooth is under the opear, 

 and almost inferior. 



SPECIES LVII. Amblema costata (Unio costata.) 



Shell somewhat thick, flattened, slightly square, 

 with large longitudinal ribs a little oblique, 

 flexuous, dilated into a wing under the ligament, 

 with curved oblique ribs; borders undulated; 

 epidermis yellowish, almost smooth ; nacre white, 

 iridescent. Breadth f diam. ^ axis -fa of 

 the length. 

 6 



