Priscinachus elongatus n. gen., n. sp., is based on four specimens from two lower Cenomanian exposures, separated by 85 km in the western Paris Basin. It is the earliest known spider crab, the Majoidea being previously known only from the lower Eocene. Priscinachus elongatus n. gen., n. sp. deserves its own family, the Priscinachidae n. fam., within the Majoidea Samouelle, 1818, due to the original combination of characters which is not found within any other majoid family. In both exposures, the fossils are preserved in a hard-ground, deposited in a shallow but open sea, in a transgressive context. Decapod fossils have been transported and often broken before deposition.