I agree with David that your animal sounds like the "angel wing",
Cyrtopleura costata.  You are pretty close to the northern extreme of the
geographic range for that species.  It is more common farther south.  If it
is the angel wing, it's a lot of work to get one.  You have to dig a deep
hole, parallel to the animal's burrow, sometimes as deep as two feet, then
tunnel in from the side until you find the critter.  If you try to dig
straight down the burrow, you will invariably hit the shell with your shovel
and break it (the shell, not the shovel).  Also, if you succeed in digging
out a live specimen, you should immediately transfer it into a bucket of sea
water, to keep it relaxed.  Otherwise the animal may contract its muscles so
strongly that it breaks its own fragile shell.  The shell can be obtained by
gradually raising the temperature of the water until the animal expires, but
don't let the water get excessively hot - again the shell is quite fragile.
An alternate method is to add epsom salts (magnesium sulfate) to the water
until the animal is "narcotized".

Paul M.