This is a beautifully-preserved juvenile ichthyosaur, almost certainly from the Lower Jurassic around Lyme Regis. The jaw is missing, probably washed away or damaged before the specimen was found and recovered. Ichthyosaurs have a body that is flattened vertically creating an oval section. That is why they are so often preserved lying on their side as is the case in this specimen.
Look carefully above the backbones and you can see the ribs on the other side of the body. The backbone is often broken into sections, because the body went from a three-dimensional structure to a flat plane as it was compressed in the sediment.
Find out more on the reptiles pages.