Palaeocybium proosti is a rare scombrid fish from the London Clay, represented by only one partial skull in the Natural History Museum, London and only a few partials in private collections. It is however possible to find isolated teeth on the beach of the Isle of Sheppey.

The teeth of Palaeocybium are the most distinctive feature. There is an outer row of minute, laterally flattened, blunt-tipped teeth and then an inner row of tightly packed, significantly larger,  laterally-flattened teeth which look similar to lateral teeth in modern Barracuda fish and also Scomberomorus (Spanish Mackerel).