122 
THE NAUTILUS. 
Atlantic drainage areas, which in the main are peopled by very dif¬ 
ferent Unio faunas. 
The species which are the subject of this note, though closely re¬ 
lated and having about the same geographical distribution, are no 
doubt distinct, and when one has learned to separate them he rarely 
has any difficulty in distinguishing them at sight. As a rule, Unio 
ochraceus is the more inflated and thinner shell of the two ; it is al¬ 
most invariably dull-colored, and the epidermis, especially at the 
posterior end is generally raised into slight irregular folds, which 
follow the growth lines. The rays, which are dirty green and ill- 
defined, usually cover the posterior part of the shell, and sometimes 
extend over the whole disc, becoming fainter towards the anterior 
Unio cariosus Say. 
end. The interior is dull-colored, often lurid, and quite frequently 
tinted with reddish salmon or purple. 
On the other hand, Unio cariosus is generally the more com¬ 
pressed of the two ; often showing a fairly well-defined angle on the 
posterior slope; it is almost invariably smooth and shining; the 
epidermis being yellowish or a dirty straw color, sometimes tinted 
with brown. Usually above the posterior slope there are a few 
wavy, very distinct, dark green rays, and sometimes, though rarely, 
this radiation extends over the shell. The interior is usually white 
or silvery, and more or less iridescent posteriorly. 
In a very large series any of these characters may fail. Speci¬ 
mens of U. cariosus may be inflated, or very rarely slightly rough¬ 
ened ; there maybe individuals of either species that are uniform 
in color, or rayed throughout, but in an extensive set they will, for 
