Mr. Barnes on the Genera Unio and Alasmodonta. 279 



Cabinets of the Lyceum, and Dr. Mitehill. My collection. 



Mr. Say's collection. 

 Diam. .9 — 1.4 Length from beaks to base 3.0 Breadth 5.0 

 Length from the top of the wing 4.3 — 4.5 

 Shell very short behind ; disks much flattened; umbones 

 depressed; beaks slightly projecting; ligament between 

 the valves ; anterior lunule much compressed and folded 

 across the transverse wrinkles; hinge margin elevated into 

 a large wing, straight and forming an obtuse angle with the 

 post, dorsal margin ; basal margin slightly rounded, nearly 

 straight ; anteriour and posterior margins somewhat angula- 

 ted ; anteriour dorsal margin arcuated, or somewhat emar- 

 ginate ; epidermis chesuut brown, glossy; surface some- 

 what deeply wrinkled and striated transversely ; slightly 

 elevated ridges and furrows diverging from the beaks to 

 the anteriour margin, and distinctly impressing the inside. 

 Teeth elevated, sulcated and radiating from the beaks; ci- 

 catrices smooth; cavity small and angular; naker bluish 

 white and iridescent; surface smooth, and polished, in old 

 specimens spotted with green. 



Remarks. — This shell resembles the Unio Alaius, in the 

 elevation of the wing and the connexion of the valves, and 

 might at first sight be mistaken for a variety of that species ; 

 but it differs in generick character, in shape, and in colour. 



* * Shells thin and small. 



4. ALASMODONTA MARGiNATA. — See Mr. Say's de- 



scription. 



5. ALASMODOJSTA UNDULATA. — See Mr. Say's de- 



scription and figure. 



These two species were the only ones known when Mr. 

 Say published his description. The former of them is very 

 common and assumes a great variety of forms and colours. 

 Those that were brought by the N. W. Expedition are lar- 

 ger than those of our eastern waters. They have the epi- 

 dermis pale green, rayed ; they are gibbous ; have the beaks 

 elevated, and base falcated. 



Diam. 1.0 Length 1.4 Breadth 2.4 



