John - For live land snails, put them in a cup of water and heat it for a 
minute or so in the microwave.  No need to boil the water.  The snail will
son hang out of the shell dead.  It can then be completely removed from the
shell with a tweezers.  Just pull gently.  This works for snalis as large 
as large mesodons down to very small stuff.  
 
Ross - I have a green abalone, one of several I collected in La Jolla years
ago, which has a dent  almost exactly like yours.  I can put the tip of a 
little finger in it.  It can be seen on the inside of the shell in inverted
form.  The abalones, as usual, have lots of small wormholes.  I have never
figured out the cause of the dent.