I picked up a small loose cobble from the shore at Ravenscar, Robin Hood's Bay. It could be locally derived or from the glacial till, but I'm left with this issue...
The ammonite, sandwiched between two pecten like bivalves is almost completely covered. It's the bivalve that I mostly need help with.
The ammonite has ribs that curve forward at the venter but don't reach the keel. The keel is eroded, but is narrow and without lateral grooves. Comparing it with a Grammoceras I have, I think it's that. If so, its upper Lias.
Looking in the Palass guide, the most similar bivalve match is Pseudopecten, but this fossil really doesn't look like one, as the radial ribbing is much finer and there are no obvious growth lines. And even if it was, it would make the fossil lower Lias.
So, please help... Is it Grammoceras and what is the bivalve?