spots is a yellow band, the tip itself being bright sangui- 

 neous. 



REFERENCE TO THE PLATE. 



The exterior, dorsal and interior views of the shell. The lower figures rep- 

 resent the branchial oviducts ; th.it of the middle is a lateral view of a portion, 

 of the natural size, as distended hy the included eggs. On the right is repre- 

 sented the summits of the tubes of wiiich this organ is composed. On the left 

 are two of the tubes magnified. 

 PLATE XXII. 



UNIO TETRALASMUS. 



SPECIFIC CHARACTER. 



Transversely elongated ; lateral teeth two in each valve. 



DESCRIPTION. 



Shell transversely oblong-elliptic ; rather thin, mode- 

 rately convex : beaks but little elevated, decorticated : 

 hinge margin parallel to the basal margin, sub-rectihnear : 

 posterior margin regularly rounded : anterior slope with 

 two impressed lines : anterior margin declining in an 

 oblique rectilinear line from the extremity of the hinge 

 margin to the subrostrated tip : basal margin subrectili- 

 near : cavity of the hinge membranes (lunulc) very slen- 

 der, obsolete : umbonial slope not prominent : within 

 white, iridescent anteriorly and on the margin ; numerous 

 minute, slightly impressed, longitudinal lines : palleal im- 



