rounded ; lunule large ; posterior margin prominent, ex- 

 tending far behind the beaks and rounded ; base a little 

 contracted in the middle ; within milk white ; teeth, a 

 single rather long, oblique, undivided primary tooth in 

 each valve ; lateral teeth rather slender, towards the tip a 

 little arquated. 



OBSERVATIONS. 



This interesting, new shell w^as sent to me by Mr. Bar- 

 abino, who discovered it opposite to New Orleans in ponds. 

 An exterior view of the shell would not immediately dis- 

 tinguish it from Alasmodonta edentula. Nob. but the in- 

 ner surface is quite different, and the armature of teeth 

 separates them genericall}'. 



REFERENCE TO THE PLATE. 



Tlie plate exhibits the cxtorior, intnrior and dorsal views of tlie slioll. 

 PI. 42, 



UNIO LUGUBRIS. 



DESCRIPTION. 



Shell transversely subovate. inflated, dark brownish or 

 blackish, wrinkled ; umbones prominent ; beaks not very 

 prominent, eroded ; ligament margin slightly arquated ; 

 ligament more or less concealed; anterior margin very 

 widely rounded, sometimes truncate or even subcmargi- 



