So what is this Cretaceous Bivalve?

 This is a bi-valve that I found at Yaverland beach on the isle of Wight about 15 years ago. It had been washed out of the soft mud about 100 metres from the tea shop. The only problem is that I can't recall which layer I thought it came from.

Now I had assumed it was cretaceous and after seeing a similar specimen in the Oxford University Natural History Museum I thought it was called Sphera corrugata  or something similar and it was located in the Cretaceous display area. However, on a recent visit to the Museum I noticed that the specimen was no longer on display. A search of the internet has produced nothing of that name and my only reference book on the period, British Mesozoic Fossils, does not include it anywhere.


I would welcome any assistance in identifying this one.