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 reflexus, 
Rar.(U. cornutus, Barnes,)and U. verrucosus, Rar. 
(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 Unios of Bayou 
Tacue are distinguished bya pearly lustre, approach- 
ing more than is usual in the American Naiades to 
the peculiar brilliancy of the pear! 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, Rar. (U. rubiginosus, Lza,) U. stramineus, 
nobeth perovatus, nob. appear to inhabit only the 
years since, found it in the Schuylkill of a purple sass 
but the finest specimens of this variety were found 12 
Chester river, Md. by Mr. Tuomas FIsHER. 
