72 OBSERVATIONS ON NAIADES, 



basal margin, roughly noduled and thick ; basal margin emarginate ; 

 substance of the shell thick ; beaks prominent, retuse ; noduled along 

 the umbonial slope and before the furrow, which is smooth ; posterior 

 slope covered with nodules ; nodules posterior to the furrow are dis- 

 posed to be transverse and on the umbones erect or recurved ; ligament 

 large; epidermis wrinkled and fuscous in adult specimens, — in younger 

 specimens it is yellowish brown with obsolete rays ; cardinal tooth 

 very large, widely cleft, sulcated and crested in the left valve, in the 

 right valve emerging from a pit ; posterior cicatrices confluent ; ante- 

 rior cicatrices distinct, the great one deep ; dorsal cicatrices situated on 

 the under part of the cardinal tooth ; cavity of the beaks angulated, 

 large and deep ; nacre beautifully pearly white and iridescent. 



Remarks. — This fine and interesting species is nearly allied to the 

 U. lacrymosus (Nob.). It differs from it distinctly in the possession 

 of nodules which are rough and disposed to be erect and transverse. 

 The tubercles of the lacrymosus take a direction towards the basal 

 margin, and are similar to tears flowing down the cheek. The pos- 

 terior margin in the present species is more protruded, while the area 

 of the anterior portion is smaller than that of the lacrymosus. It 

 cannot be mistaken for the U. metanever (Rafinesque), which possesses 

 large elevations along the umbonial slope. In younger specimens than 

 the one represented here the basal and posterior margins are more 

 rounded. 



Unio CoNGARiEUS. Plate VI. fig. 4. 



Testdrhomboideo-ellipticd, transversa, incequilaterali ; valvulis tenuibus; natibus 

 subundulatis; dente cm'dinali obliquo compressoque ; dentibus lateralibiis longis, et 

 prope terminos posteriores auctis ; margaritd sericea et iridescente. 



Shell elliptico-rhomboidal, transverse, inequilateral ; valves thin ; beaks slightly 

 undulated ; cardinal tooth oblique, compressed; lateral teeth long and enlarged 

 towards the posterior end ; nacre satinlike and iridescent. 



Hab. Congaree River, South Carolina. 



