placing a crocea clam

I placed mine on a rock and he looks like he is going to fall off... I looked underneath him and it looks like he actually ground a V in the rock... is that possible?
 
they are supposed to be able to make holes in limestone with and acid that it excretes out of the back but I am not positive.
 
They are a very shallow water clam, so it is good to place them up high as well ;)
 
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