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sometimes confluent with the solitary impression 

 on this side of the shell, is often distinct from it. 

 If this genus differs in the animal, from the Unios 

 of Europe, it must be called Elliptic, a name which 

 I have given to one of its sub-genera. I divide it 

 into four sub-genera. 



First Sub-genus ELLIPTIC. 



Shell elliptical ; axis extramedial ; cardinal 

 tooth furrowed ; contour thickened ; ligament 

 corneous ; lamellar tooth straight. 



SPECIES I. Unio Nigra (Elliptio Nigra.) 



Shell oval-elliptical, slightly ventricose, (bom- 

 bee /) with a slight angular truncation posteriorly ; 

 thick ; epidermis blackish ; nacre roseate ; lam- 

 ellar tooth thick, obtuse, slightly crenulated. 

 Length T 9 j diam. T 6 j axis % of the breadth. 



This is one of the large species produced in 

 the Ohio, sometimes measuring six inches in 

 breadth ; nacre fine, frequently iridescent, as are 

 always the muscular impressions ; fossule distinct. 



The following description of the form of the 

 hinge, applies to all the species. Cardinal tooth 

 bilobate, anterior, thick, trihedral, furrowed, lobes 

 of unequal size, the anterior smallest in the right 

 valve, the opposite one in the left ; lamellar tooth 



