UNIO 651 
Group of Unio complanatus. 
Shell elongate trapezoidal, moderately solid, but little inflat- 
ed, nearly straight below; with a posterior ridge, and usually 
biangulate behind; beaks not prominent, sculptured with sev- 
eral coarse, parallel ridges, which follow the growth lines or 
are nearly straight. Epidermis sometimes shining and feebly 
rayed, becoming rough and rayless with age; laterals straight 
or slightly curved; beak cavities shallow. 
Animal with the marsupium occupying all or nearly all of 
the outer branchiz; gills long, rounded below, inner much the 
larger, free nearly or quite their whole length from the ab- 
dominal sac. 
Unio COMPLANATUS “Solander” (Dillwyn). 
Shell generally rhomboid, inequilateral, subsolid to solid, 
convex; anterior end rounded; dorsal and basal lines nearly 
or quite parallel; dorsal slope obliquely truncated; posterior 
ridge well developed, single or double, ending behind at or 
near the base in a point or biangulation ; beaks not much raised 
nor inflated, their sculpture consisting of strong ridges, which 
run nearly parallel with the growth lines and are carried back 
to the nucleus behind as delicate, radial lire; surface with ir-. 
regular growth lines, often nearly smooth in young or well 
developed shells, rough in old ones; epidermis tawny-green to 
greenish-brown, often rayed and rather smooth in young shells, 
becoming rough and dark in old ones; left valve with two 
ragged pseudocardinals and two nearly straight laterals; right 
valve with one pseudocardina!l and often a small one above it 
with one lateral; beak cavities shallow ; muscle scars large, im- 
pressed: nacre white, straw-color. salmon or various shades of 
purple. 
Length 105, height 55, diam. 33 mm. 
Length 9o, height 55, diam. 30 mm. 
Length 60, height 38, diam. 21 mm. 
Length 62, height 40, diam. 25 mm. 
