Holotype specimen of <i>‘Plesiosaurus’ macrocephalus</i> in the Natural History Museum, London. Photo by Tom Challands. 
Cast of the holotype of <i>‘Plesiosaurus’ macrocephalus</i> 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.