Unknown British Jurassic fossil

[Aurelius]

Can anybody please ID this? Found it on the Somerset coast, in the UK. Although the rocks there span from the Triassic to the Jurassic, I believe this is probably Jurassic, because rocks with this appearance sometimes contain ammonites. 

 

 

[Ludwigia]

Looks like a bivalve shell at first glance, but I couldn't say which one. Can you prep it out some more?

[Aurelius]

Unfortunately my compressor is out of action at present, but once I have a new one I'll give it a go.

[TqB]

First guess is a Modiolus (M. scalprum would fit, for example) but I agree we need to see more of it.