Hitherto the bivalve fauna of the Spilsby Sandstone and contiguous deposits including the lower part of the Sandringham Sands and Speeton Clay of-Middle Volgian to Ryazanian age has been inadequately described. As a consequence of this study 92 taxa have been recorded. 45 of these are described fully and include 13 new species and one new subgenus. Information has been obtained from the author's collecting and from museum' collections. Locality and section information is given for sites examined by the author. The preservation of the fauna is normally as moulds, but cold cure silicone rubber has been used to obtain casts with great success. The ecology of the less well understood bivalves is discussed. Five bivalve dominated assemblages are recognised. They represent various shallow marine facies. The sediments include glauconitic sands and silts with condensed sequences containing phosphatised nodules and also a sideritic ironstone occurs. A facies model is produced for the East Midlands Shelf, and the contemporary relationships with other parts of England are discussed. The Spilsby basin represents a westerly embayment or estuary of the Southern North Sea basin. In Middle Volgian times there were marine connections to the Wessex basin to the south, but with the subsequent draining of this latter area in Upper Volgian to Ryazanian times to restricted marine, brackish and freshwater lagoonal environments, open marine connection ceased. The Spilsby basin remained fully marine during these times, but became more enclosed. The Spilsby bivalve fauna indicates strong faunal similarities with Boreal regions, and in particular with East Greenland and the Russian Platform. The fauna also compares closely with the sandy Upper Kimmeridgian facies in central England but contrasts strongly with the contemporary faunas of the Portland and Purbeck Beds. The relationships of the Spilsby fauna with other Boreal regions and with Tethyan Europe are discussed.