I've saw thousands of Buccinum undatum and Neptunea lyrata decemcosta in
my collecting days from the scallop trawlers, and not a one was
sinistral: it must be an exceedingly rare occurance!  I did hear of a
sinistral undatum from Norway once, however, so it DOES happen, just not
more often than once in the proverbial Blue Moon.

From stormy old New Scotland,
Ross Mayhew.