172 Thikty-eifth Keport 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 (PI. 11, fig. 1 , a. r.J, 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 (PL 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 aline, corresponding 

 in curvature 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. (Pl.ll, 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 a shell ; 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, 



