10 
the mutability of colour, and presents every shade 
of salmon colour and purple, whilst other specimens 
are pure white; others again bluish and highly irri- 
descent. Amongst those hitherto found only white 
in the nacre, I can show examples of Unio rejiexus , 
Raf. ( U. cornutus, Barnes,) and U. verrucosus , Raf. 
( U tuberculatus , Barnes,) which are purple. I found 
five specimens of the former in the Black Warrior 
river, near Erie, and three in the Alabama river, all 
purple; four of them a deep shade of that colour* 
They accompanied others of the same species, per- 
fectly white. I found at Erie only one specimen of 
U. verrucosus , which is purple, and the tint is pale. 
The colour of most species is apt to vary in the dif- 
ferent rivers. Thus most of the IJnios of Bayou 
Teche are distinguished by a pearly lustre, approach- 
ing more than is usual in the American Naiades to 
the peculiar brilliancy of the pearl oyster. 
Some of the small streams in Alabama contain 
shells that I have not found in the rivers to which 
they are tributary. Thus Unio declivis , Say, U- 
flavus , Raf. ( U. rubiginosus , Lea,) U. stramineus , 
nob. U. perovatus , nob. appear to inhabit only the 
years since, found it in the Schuylkill of a purple colour* 
but the finest specimens of this variety were found in 
Chester river, Md. by Mr. Thomas -Fisheb. 
