37 
founded under the general name of purpureus. Mr. 
Mason, who has more thoroughly investigated these 
shells than any other Conchologist, has brought 
together in his cabinet specimens so dissimilar in 
appearance as to perplex the student if he admits 
them as varieties only of one species; for in that case 
he must extend his scepticism to a number of other 
shells, hitherto admitted as distinct species. Differ- 
ent localities, no doubt, produce some corresponding 
difference in the general appearance of certain shells, 
_a remarkable instance of which is exhibited in U. 
niger, Rar., which in Flint river is quite a dwarfish 
shell, compared with the same species in other 
rivers. This remark will apply equally to U. costa- 
tus, Rar. of Flint river; but the specific character is 
not lost in the varieties of these two shells, which 
cannot be said of the so called varieties of U. purpu- 
reus. Perhaps U. coarctatus, Lam. will prove a dis- 
tinet species, but I leave it for a more accurate ob- 
server to make further comments on this difficult 
subject. Noone could do it more justice than Mr. 
Mason. 
UNIO PECTOROSUS. 
Puiate VI.—Fig. 1. 
DESCRIPTION. 
Shell oval, ventricose, not thick; posterior margin 
broad, biangulated; umbonial slope subangulated; 
space behind flattened, with an impressed line passing 
from the beak to near the middle of the posterior 
margin; anterior side slightly produced; umbones 
D 
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