wide and the radii are interrupted into spots. It may prove 

 to be a distinct species when more specimens are obtained 

 for comparison, and it may in that case be distinguished 

 by the name oi perplexus. For a specimen of it I am in- 

 debted to Mr. Lesueur. 



REFERENCE TO THE PLATE. 



The upper figure exhibits a front view of the full grown ehell. 



UNIO MONODONTA. 



SPECIFIC CHARACTER. 



Transversely elongated ; inner margin whitish or livid ; 

 lateral teeth obscure. 



DESCRIPTION. 



Shell transversely much elongated, slightly arquated, 

 blackish-brown ; umbonial slope (Swainson) widely round- 

 ed, not abruptly elevated : umbo placed far back, not pro- 

 minent above the general curvature ; hinge and basal mar- 

 gins a little arquated, parallel : anterior and posterior mm- 

 gins regularly rounded : ivithin perlaceous, witli an opake 

 whitish or livid margin : primary teeth simple, small : late- 

 ral teeth very imperfect ; that of the right valve with only 

 an impressed, short line, instead of the usual deep recipi- 

 ent fissure. 



PLATE VI. 



