Today I found this strange fossil thing at Corby Glen, Lincolnshire, whilst digging through a soily bank in the garden. It is an asymmetrical heart shape with grooves running over the top in a V-shape parallel to the point. There is a deeper groove on the top running from the point to the indent of the heart. Approx 4ins x 3ins x 2ins. The grooved top has remnants of a grey layer and the underside seems to be made up of crushed shells. Could it be part of a large tooth; the grooves used for grinding (molar?)?
It's definitely a bivalve, with both valves of the hinged bivalve shell fossilised together. Bivalves are shelled marine and freshwater creatures like clams, mussels etc. Judging by the age of the rocks where you found it, it is probably lower upper Jurassic in age, so perhaps around 160 million years old or just less. It's from the heyday of the time of the dinosaurs. Judging from the shape it could be something like "Pholodomya". it's hard to be sure from these photos.
If you could take some nice clear, well lit photos showing the fossil from all angles (top, bottom, back, front and both side), we may well be able to tell you more about it.
All the best,