I find that a vernier-type caliper is the most suitable for measuring
shells, but that doesn't mean you have to spend big bucks.  You don't need
something made of special steel alloy and guaranteed accurate to .01 mm.
Inexpensive plastic verniers are available, accurate to 0.1 mm, which is
good enough for most conchological purposes.  The plastic ones I have seen
have all been the same size - 150 mm maximum gape.  They come in two
versions, those which read directly from a linear rule (about ten dollars)
and those which read from a rotating dial (about twenty dollars).  The only
problem you might encounter in measuring pecten shells with these is that
the length of the jaws is about two inches, so on a shell more than four
inches in diameter, you would not be able to reach the center of the valves
to measure "thickness".  However, this would also be true with an expensive
stainless steel caliper the same size.  Larger calipers are available, but
not in plastic.  I have one with 300 mm capacity and four inch jaws, but
it's a steel one and cost me over $100.00.
Paul M.