173 



near their tip ; beyond these spots is a yellow band, the tip itself 

 being bright sanguineous. PI. 22. The exterior, dorsal and in- 

 terior views of the shell, and the branchial oviducts. 



Unio tetralasmus. — Spccijic character. Transversely elon- 

 siatcd ; lateral teeth two in each valve. 



Desc. Shell transversely oblong-elliptic; rather thin, moder- 

 ately convex ; beaks but little elevated, decorticated ; hinge mar- 

 gin parallel to the basal margin, subrectilincar ; posterior margin 

 regularly rounded ; anterior slope with two impressed lines ; ante- 

 rior margin declining in an oblique rectilinear line from the ex- 

 tremity of the hinge margin to the subi'ostrated tip ; basal margin 

 subrectilincar; cavity of the hinge membranes (lunule) very slen- 

 der, obsolete ; umbonial slope not prominent ; within white, irides- 

 cent anteriorly and on the margin; numerous minute, slightly 

 impressed, longitudinal lines ; palleal impression not indented, 

 curved beyond the anterior muscular impression; anterior accessory 

 muscular impression very large, separate; cardinal teeth trans- 

 versely long, slender, very oblique ; lateral teeth distant from the 

 cardinal teeth, two in each valve, equally prominent in the two 

 valves. 



Obs. For an opportunity to examine this species I am indebted 

 to Mr. Lesueur, who obtained it in Bayou St. John, near New 

 Orleans. It is readily distinguished from any other species. PL 23. 



Unio dehiscens. — Specific character. Anteriorly biemargin- 

 ate; teeth obsolete. 



Unio dehiscens, nobis. Disseminator of Useful Knowledge, vol. 

 2, p. 308. 



Desc. Transversely much elongated, thin, rather compressed, 

 polished, pale yellowish green, or brownish, radiate with some- 

 what interrupted green lines, which are more or less dilated and 

 compound, and obsolete in the aged specimen ; anteriorly gaping, 

 and at the posterior inferior margin also gaping to the middle of 

 the base ; a slightly impressed line on the anterior hinge submar- 

 gin passes to the anterior margin, which is emarginate both above 

 and below the line, and subangulated at tip : posterior margin 

 rounded, extending considerably beyond the beaks : umbo and 

 beak not prominent, with slight undulations on the latter : primary 

 teeth consisting of a slightly elevated, very obtuse angle in the 

 right valve and obsolete sinus in the left for its reception : lateral 



