AND DESCRIPTIONS OP NEW SPECIES. 117 



in the possession of two remarkable curved furrows on the umbonial 

 slope. The specimen represented in the engraving is chosen on ac- 

 count of its being adult, although the beaks are not in a perfect state of 

 preservation. The enlargement and dentate appearance of the poste- 

 rior margin is very remarkable in this and some other of the species. 

 It occurs more frequently in the sulcatus (Nobis), and has been par- 

 ticularly noticed in the remarks on that species*. This variety of , 

 sulcatus has been considered by Mr Say as a distinct species, to which 

 he has given the name of ridibimdus. I have never thought that it 

 could be considered to differ specifically from the sulcatus. 



Unio Subrotundus. Plate XVIII. fig;. 45 



»• 



Testa suborbiculata, subventricosa; valvulis crassis; natibus prominentibus; epi- 

 dermide circa nates luted, juxta mar ginem fused; radiis interruptis; dentibus car- 

 dinalibus crassis, lateralibus subcurvis brevibusque; margaritd alba et iridescente. 



Shell suborbicular, subventricose ; valves thick ; beaks elevated ; epidermis yel- 

 low about the beaks, brown towards the margin ; rays interrupted; cardinal teeth 

 thick ; lateral teeth short and slightly curved ; nacre pearly white and iridescent. 



Hab. Ohio, T. G. Lea. 



My Cabinet. 

 Diam. 11, Length 1-6, Breadth 1-6 inches. 



Shell suborbicular, nearly equilateral, subventricose; substance of 

 the shell thick, somewhat thinner behind ; beaks thick and elevated ; 

 ligament rather short and thick ; epidermis yellow and smooth in the 

 region of the beaks ; brown and finely wrinkled towards the margin ; 

 interrupted rays pass from the beaks and are very visible over the um- 

 bones, but are lost in the wrinkles before they reach the margin ; car- 

 dinal teeth thick and crenate; lateral teeth short, thick and very 

 slightly curved ; posterior and anterior cicatrices both distinct ; dorsal 



* See Vol. III. p. 431. 



Vol. IV.— 2 E 



