172 THIRTY-FIFTH REPORT ON THE STATE MUSEUM. 
On the interior of the shell, near the anterior dorsal margin, is seen 
a comparatively large oval marking, caused by the attachment of the 
anterior adductor muscle (PI. 11, fig. 1, a.a.); just posterior to this are two 
other impressions, one at the upper and the other at the lower por- 
tion ; the upper one is caused by the attachment of the anterior retractor 
muscle (Pl. 11, fig. 1, a. r.), the lower one by the attachment of the 
protractor pedis muscle (Pl. 11, fig. 1, p.p.), and are known respectively 
as the anterior retractor and the protractor pedis muscular impres- 
sions. 
At a short distance from the post-dorsal margin, near the angle 
formed by the junction of the hinge-line and posterior slope, is another 
large marking formed by the attachment of the posterior adductor 
muscle (P]. 11, fig 1, p. a.), and is known as the posterior adductor 
muscular impression. Immediately anterior to the upper portion of 
this impression, is a smaller nearly circular impression, formed by 
the attachment of the posterior retractor muscle and is known as the 
posterior retractor muscular impression, (Pl. 11, fig. 1, p. r.) Extend- 
ing from the umbo to each adductor muscular impression, is a faint, 
gradually enlarging marking, caused by the adductor muscles, 
forming continuous impressions as they changed their position 
with the advancing growth of the shell. (Pl. 11, fig. 1, mar.) Connecting 
the posterior and anterior muscular impressions is a line, corresponding 
in curyature with the ventral margin of the shell, known as the pallial 
line, and formed by the attachment of the mantle to the shell by means 
of numerous muscles along this line. (Pl. 11, fig. 1, p.1.; Pl. 5, fig. 1, 
p- 1.) Near the umbo are several impressions caused by the attach- 
ment of adductor muscular fibres. (PI1.11, fig. 1, m.f.) 
The thickness of the shell varies in different localities, even when only 
a short distance apart, and does not depend upon the amount of lime 
(of which substance the shell is principally formed) in the water, 
but upon the power of the animal to absorb, and assimilate it 
into ashell; thin fragile specimens being found in water, rich in 
lime, and others more massive, in waters where that material is 
much less abundant. That the thickness of the shell is not due to 
the amount of lime in the water is shown by the fact that several 
species occur in the same stream, some having massive and others 
fragile shells; Unio undulatus having a shell one-fourth of one inch 
or more in thickness, and Unio gracilis with a shell one-twentieth of 
an inch in thickness, occur in the same locality. The conditions 
favorable for the development of one species may be unfavorable for 
the development of another, even of the same genus. In the canal at 
West Troy, N. Y., specimens of Anodonta implicata are found of 
very large size, the shells free from erosion, while Anodonta fluviatilis, 
