FRESH-WATER MUSSELS. 131 



laminae but must have some other significance. They probably mean nothing more 

 than slight and frequent but irregular retractions of the margin of the mantle during 

 the process of shell formation, which have registered themselves in fine wrinkles on 

 the surface of the shell as it is built. The more conspicuous rings that mark some 

 shells still await our attention. 



Fig. 3. — Sections Uirough prismatic layer of Quadrula ebenuj. The sections were made at different levels, the prisms being smaller 



and more numerous in the outer portion. X 300. 



Growth of the shell in length and breadth is accomplished by the secretion of shell 

 substance of the three layers by cells at or near the margin of the mantle. There are 

 certain cells of a furrow in the margin of the mantle which form only periostracum, 

 and there is a certain portion of the mantle near the margin which forms only prismatic 

 shell substance, while the greater portion of the mantle surface normally forms only 

 nacre. Now, the; important point for our present consideration is this: If, from any 

 cause, the margin of the mantle is made to withdraw within the shell to such an extent 

 as to break its continuity with the thin and flexible margin of the shell, then, as the 

 study of sections indicates, when the deposition of shell is resumed, the new layers 



Fig. 4. — Section through the interruption ring on pocketbook mussel, caused by handling mussel 

 in summer. Simple duplication. 



of prismatic substance and periostracum are not continuous with the old, end to end, 

 but are more or less overlapped by the old. In other words, growth does not begin 

 again exactly where it left off, but a little distance back therefrom, and the cause of this 

 is largely mechanical (text fig. 4). The amount of overlapping probably depends 

 upon the degree of disturbance and the extent to which the mantle has withdrawn 

 itself. The result is an unwonted duplication of layers. Counting inward from the 



