328 Variation in Shells. [ ZOE 
The moulding influence of the currents, sand and gravel, is plainly 
seen on the upper portion of the shell (fig. 6), which is admir- 
ably adapted to the purpose it serves and is strong evidence of its 
sedentary habits, and to its incurving form is largely due the 
prominence of those remarkable ridges or angles that constitute 
the principal or distinguishing characters of the shell. This in- 
curving then, of the upper part of the shell, would press down and 
crowd the anterior portion of the body and compel its lateral ex- 
pansion, then a corresponding enlargement of the shell would fol- 
low, while the compact gravel on the outside acting.as a sort of 
fulcrum would bring the pinch just about where these angles occur. 
To sum up the matter then, we must suppose that Anodonta Ore- 
gonensis, for some cause, has changed its habitat and station, from 
the quiet or still waters of the pond or lake, to the rapid flowing 
mountain stream. 
Being unable to contend against the force of the current, it bur- 
- rows in the compact sand and gravel for safety. The flowing waters 
bring it an abundance of food, dispensing with the necessity of 
moving around to secure it, and thus by force of circumstances it 
becomes a sedentary river mussel, while the factors in its environ- 
ment contending with the organic energy of the mollusk result in 
a modified shell. 
Anodonta Nuttalliana var. Idahoensts (figs. 3-4). 
In general outline this variety is more oval than any of the other 
forms of our West Coast Anodons. It occupies a position between 
A. Californiensis and A. Oregonensis, and is one of those interme- 
diate forms that are constantly occurring and shake our faith in what 
we call species. 
In the proportions of its length and height and its rounded 
posterior outline, it resembles Anodonta Californiensis. In its breadth 
between valves, and the smaller and more pointed anterior end of 
the shell, and more especially in the less developed dorsal preva 
tions of the valves, it resembles A. Ovegonensis. 
In color and lines of growth it agrees fairly well with both of the! re 
other species. In the young state the shell is higher in propor- 
tion to its length, while the wings or dorsal prolongation of the 
valves are a little more produced. The umbones of the very young 
of all our West Coast forms, until they have attained about a-half 
inch in length, present a peculiar wavy or corrugated appearance, a 
