606 UNIO 
different, that of trapesoides though nodulous in an early stage 
becomes obliquely plicate on the disk as the shell approaches 
maturity, while that of sloatienws is much finer and nearly ver- 
tical. The beak cavities of the latter are much shallower than 
those of trapezoides and it lacks the scars in the cavities of 
the shell, and finally the anterior scars of the latter are very 
rough while those of sloatianus are smooth. A little attention 
to these characters will enable any one to separate the two 
species without difficulty. 
Lea’s type of sloatianus is an old, arcuate shell, while the 
type of atromargimatus is young and quite different looking. 
But the young plicate atromarginatus gradually changes into 
the smoother sloatianus as it grows older, and specimens of 
both show the dark bordered nacre, though it is more pro- 
nounced in the young shells. 
Group of Unio crassidens. 
Shell generally solid, rather inflated, rhomboid oval, with a 
sharp, well-developed posterior ridge; beaks prominent, the 
sculpture consisting of a few coarse ridges running nearly 
parallel with the growth lines, and swollen at the posterior 
ridge; surface of the valves nearly smooth or only marked 
by growth lines; posterior slope bearing wrinkled, corrugate, 
radiate ridges; epidermis often faintly rayed. 
Animal having the branchize very large, rounded below, 
inner the larger, free nearly or quite their entire length from 
the abdominal sac; branchial and anal openings large, with 
many small papille; marsupium occupying the entire outer 
gills. 
UNIO CRASSIDENS Lamarck. 
Shell ponderous, convex or subinflated, subrhomboid, inequi- 
lateral; beaks full and elevated, their sculpture a few coarse 
ridges running nearly parallel with the growth lines; posterior 
ridge well developed and angled, sometimes faintly double 
below and ending behind at the base of the shell usually in a 
biangulation : base line straight or incurved in old specimens; 
