702 UNIO 
This differs from U. popeii in several important particulars 
It is more nearly cylindrical, its greatest diameter being he- 
hind the middle. but it scarcely diminishes in front of that; 
its nacre is lighter colored and the beaks are much smoother. 
Section CANTHYRIA Swainson, 1840. 
Canthynia Swainson, Treat. Mal., 1840, p. 378. 
Shell inflated, suboval, spinose, with a high, rather sharp 
posterior ridge, above which it is somewhat truncated, the 
posterior slope being sometimes slightly wrinkled ; beaks rather 
compressed, the sculpture not seen; epidermis smooth and 
shining, variegated with angular blotches; hinge sharply 
curved at the center; pseudocardinals rather compressed ; lat- 
erals short, remote, the hinge plate narrowed; beak cavities 
rather deep. Animal with the marsupium occupying the whole 
of the outer gills, forming a smooth pad; branchiz very large, 
round below, inner the larger, free nearly their whole length 
from the abdominal sac ; palpi large ; mantle double on its edge, 
sometimes with a few papille in front of the branchial open- 
ing; branchial opening small; superanal opening colored 1n- 
side. 
Type, Unio spinosus Lea. 
UNIo SPINOSUS Lea. 
Shell subrhomboid or subtriangular, subinflated to inflated, 
rather solid, inequilateral or almost equilateral; beaks high 
and full, their sculpture apparently a few short, irregular cor- 
rugations; posterior ridge well developed, decidedly angled, 
ending behind in a point at or just below the median line; 
anterior end rounded; dorsal line sharply curved at the beaks; 
dorsal slope obliquely truncated; base line curved; surface 
with very fine, concentric sculpture; epidermis tawny-yellow, 
green or brownish, usually with faint and beautiful rays, shin- 
ing; left valve with two rough, subcompressed peudocardinals 
that are nearly opposite each other, with two short, club- 
shaped, remote laterals; right valve with two opposed pseudo- 
cardinals, the upper smaller, with one high lateral: hinge line 
