38 MOLLUSC A. 



In removing a valve observe the same cautions as 

 in the case of the oyster ; but observe that here two 

 muscles (Fig. 3, aa,pa), one at each end, are to be 

 separated from the shell ; and the animal also has a 

 thick muscular border to the mantle, as well as smaller 

 muscles near the hinge. 



If successfully removed, the clam will appear as in 

 Fig. 3, and by contrast with the oyster the following 

 facts may be elucidated. First, that the mantle is a 

 bag with thick, fleshy borders, which, instead of being 

 free all around, are closely attached. Then, if a probe 

 be passed along the depression which marks the junc- 

 ture of the fleshy border of one leaf of the mantle with 

 that of the other, it will slip near the larger end into 

 an opening (Fig. 3, op) . This is the only orifice except 

 the two at the ends of the funnels. The two large 

 muscles, and the thickest muscles of the mantle border 

 and of the base of the funnels, and the impressions they 

 make on the shell, are important. They enable the 

 observer to see at once that the clam builds the shell 

 more with the border of the mantle, than with the part 

 which lies inside of the limits of the impressions made 

 by the muscles, and that therefore a valve is not so 

 thick as in the oyster, and is scoop-shaped in conse- 

 quence (Fig. 49) ; in other words, the layers are 

 added very fast in the outer or border region {es) of 

 the shell, and very slowly inside {is) ; whereas, in the 

 oyster, the inside is also built up rapidly (Fig. 46). 



This also accounts for the fact that the beaks of the 

 valves or the young shells of the clam are apparently 

 closer together, but really quite as widely separated, if 

 the layers are counted, as in the oyster. A marked 



