2nd photo has siphons
No interactions present.
As you're both so good at identifying, D.v's, thought you might also be interested in the challenging "can you tell a standard mustard-coloured Donax vittatus from a Moerella donacina" at http://www.ispot.org.uk/node/110992 (there's links onsite). It's on a par with the whelk-dogwhelk challenge. I've added naturalhistory.museumwales.ac.uk links, which has comprehensive Uk's marine bivalves.
for a more interesting puzzle Nick, have you seen nightfly's prawnskins at http://www.ispot.org.uk/node/290209 ? The 1st seems to have cricket bats for arms.
Lat/Lng: 51.5758, -4.2969
OS grid ref: SS409888