A large and previously unknown ichthyosaur is reported from the Kimmeridge Clay (Upper Jurassic) of Stowbridge, Norfolk, England. The only valid genera described from the Upper and Middle Jurassic are considered to be Ophthalmosaurus Seeley and Nannopterygius von Huene, and since the Stowbridge specimen is distinct from these, a new genus and species, Grendelius mordax gen. et sp. nov., are erected. Using cluster and principal coordinates analyses, G. mordax was found to have closest phenetic affinities with Platypterygius americanus (Nace) of the North American Cretaceous, and with an undescribed ichthyosaur from the Middle Triassic of Switzerland, and least phenetic affinity with Ophthalmosaurus discus (Marsh).The phenetic analysis also revealed that the division of the Ichthyosauria into latipinnates and longipinnates is questionable. Stenopterygius quadriscissus (Quenstedt), long considered a longipinnate, is herein shown to have closer phenetic affinity with latipinnate ichthyosaurs. The close phenetic affinity between the skulls of P. americanus and the undescribed Swiss ichthyosaur is inconsistent with currently held views of cranial evolution in ichthyosaurs, and points to the shortcomings of tracing evolutionary trends in temporal sequences of fossils.