Holotype specimen of ‘Plesiosaurus’ macrocephalus in the Natural History Museum, London. Photo by Tom Challands. 
Cast of the holotype of ‘Plesiosaurus’ macrocephalus in the Bath Royal Literary and Scientific Institute. Photo by Adam S. Smith.
The holotype specimen (NHMUK PV R 1336) was first and last described by Owen, 1840. ‘Plesiosaurus’ macrocephalus is probably a juvenile rhomaleosaurid, and certainly doesn’t belong to the genus Plesiosaurus. However, pending revision of the taxon and referral to a different or other existing genus, it is still named ‘Plesiosaurus‘, following Owen.