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 shelly and the branchial oviducts. 
Unio tetralasmus,. — specific character. Transversely elon- 
gated ; 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, subrectilinear ; 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 subrostrated tip ; basal margin 
subrectilinear ; cavity of the hinge membranes (Innule) 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. 
Ohs. 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. PI. 23. 
Unio dehiscens. — -Specific character. Anteriorly biemargin- 
ate; teeth obsolete. 
Unio dehiscensj nohis. Disseminator ofi 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 : primaiy 
teeth consisting of a slightly elevated, very obtuse angle in the 
right valve and obsolete sinus in the left for its reception : lateral 
