The specimen NHMUK 39514 comprises the cranial remains of a plesiosaur found on the Dorset coast during the middle of the ninetieth century. It was partially described and figured by Owen and variously attributed to Plesiosaurus rostratus (Owen) and to Plesiosaurus conybeari (Sollas). NHMUK 39514 is a partial braincase and mandible. It differs from all other Early Jurassic plesiosaurs known including the two species to which it was originally referred. The mandible presents several pliosauromorph characters but the specimen is identified as Plesiosauria indet. because it has several plesiomorphic characters in its braincase anatomy. This assignment will prevent the introduction of a possible chimera in future phylogenetical analyses and paleobiogeographic reconstructions.