154 



the position of the beaks varies a little. The umbonial slope is 

 often elevated into a rib, and the striae occupy a considerable part 

 of the shell and become so profound as almost to denticulate the 

 edge of the shell. PI. 4. 



Unio sulcatus, Lea. — Specific character. Transversely sub- 

 quadrate ; posterior margin not extended beyond the umbones. 



Unio sukatus, Lea. Trans. Amer. Philos. Soc, vol. 3, N. S. 



Desc. Shell very obliquely transverse, subquadrate ; with an 

 indented groove extending from the umbo to the anterior and 

 anterior basal margins, between which the edge is somewhat re- 

 tuse ; epidermis yellowish-olivaceous, with radiating, deep green, 

 more or less undulated lines ; summit near the posterior extremity, 

 prominent ; anterior margin with two obtusely rounded angles, a 

 little gaping ; cavity of the hinge membranes behind the summits 

 deeply excavated, nearly as long as wide, distinctly angulated be- 

 hind ; posterior margin regularly rounded, remarkably short, hardly 

 extending beyond the line of the posterior tip of the umbo ; within 

 perlaceous-white, more particularly iridescent before ; teeth regular, 

 anterior cardinal tooth parallel with the lamelliform tooth, which 

 is very slight arquated ; posterior muscular impression deep ; sinus 

 of the cartilage very slight, regular ; cavity of the umbo not deep, 

 its muscular impressions obvious; palleal impression somewhat 

 truncate before, and not extending anteriorly beyond the line of 

 the muscular impression. 



Var. a. Within, except on the anterior margin, purple or livid. 



Ohs. It occurs in the Wabash and Ohio rivers. In order to 

 avoid confusion, I may mention that the present species is in many 

 of the European collections, sent by me within the last five years, 

 under the name oi fiagellatus. PI. 5. 



Unio RIDIBUNDUS. — Specific character. Subquadrate ; emar- 

 ginate before and denticulated on the anterior basal angle. 



Desc. Shell transverse, subquadrate oval, olivaceous, radiate 

 with capillary deep green lines ; summits somewhat prominent, 

 nearly terminal : cavity of the hinge membranes oval, rather larger 

 behind the beaks than broad : posterior side veiy short : anterior 

 side with a groove, obsolete on the umbo, but becoming deeper to 

 the anterior edge, which is consequently emarginate a little below 

 its middle; a little gaping; the inferior angle somewhat more 

 prominent, rounded and denticulated : anterior muscular impression 



