H, firstly I did have to look at tweak etc your photos several times before I made my mind up! The turtle bone looks pretty classic to me but I am always open to 'peer review' as it were!
I would do a search using diferent terms including 'turtle',' carapace' location found etc - but there's lots out there to sift through!
Ah, now that I know more about 10 it should be with 12 & 13.but does ave a bit of sell in it! Without having it in my hand it is a bit of an enigma - are you able to take it somewhere NHM - Angela Mamont Centre, good regional museum etc)? Just seen where you are based - Oxford?
Thanks again for having another look at them. I will have a look at the Centre and see if it's local enough to take it in. I will also do a bit of research as suggested. Hopefully I'll come back with some more information