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THE POPULAR SCIENCE MONTHLY. 



The first, on the dorsal face (Fig. 3), is occupied in part by a pal- 

 lial prolongation, a continuation of which is introduced by this opening 

 into the interior of the shell, in part by the small adductor muscle. 



Fig. 3. 



Fig. 4 



The second opening is posterior (Fig. 4, a), and serves to open a 

 passage to the internal organs contained in the cavity of the mantle. 



Finally, the third, placed obliquely in front (Fig. 5, a a; Fig. 4, ), 

 is the largest, and remains always gaping ojDen to allow the foot to pass 

 out (Fig. 5, 1). 



Each of these valves, which form the shell, is formed of three parts, 

 viz. : 



Fig. 5. 



Fig. 6. 



1. A posterior part (Fig. 3,/; Fig. 6,f; Fig. 7/), which we can call 

 the neck part ; this posterior is the least arched, and thinner than the 

 rest of the shell: its posterior edge is embraced by the folds of the 

 mantle, which we have already mentioned, and thus the mantle is sol- 

 idly attached to the shell. 



