A new turtle Mesochelys durluonensis gen. el sp. nov. is described from an almost complete skull and partial postcranial skelelon found at Durlston Bay, Dorset (Purbeckian, basal Cretaceous). Its closest relative is Glyplops plicalulus from the Morrison Formation of North America and, together, these two constitute the mosl primitive cryptodire family Glyplopidae. The structure of the basicranial axis, the pterygoid, and the roofing bones indicate that Mesochelys is the most primitive post-Triassic chelonian yet described.