Now here's a find (by my better half). There are millions of them 15 miles up the coast at Redcar but they are relatively infrequent finds at Runswick. This one has been tumbled around in beach pebbles for a while. It's a bivalve, a jurassic oyster relative and you usually find the curved lower shell which gives the fossil its nickname, but this one has some intriguing structures fossilisised on the gap where the flat upper shell would be: look for the scallioped structures which plug the usually hollow open end of the lower shell. If any Gryphaeaologists can comment, I'd be grateful. There is too much of the scalloped edge visible for it simply to be the upper valve of the Gryphaea.
Here's the thing sideways on:
Gryphaea arcuata (Lamarck 1801) found lower Jurassic. They were believed to relieve rheumatism in humans and sore backs in horses.