Detailed examinations of two hitherto unknown collections of ophthalmosaurs have been made. Highly ossified specimens have been used to make a restoration of the back of the skull and to bring the occipital and otic regions into their correct relationships with the roofing bones of the skull. The bony labyrinth has been re-interpreted. The frontal bone is described in detail. Details of post-cranial osteology are also given, including the presence of a posterior notch in the coracoid of O. icenicus Seeley. This species is rare, most of the known specimens being allocated to the species O. monocharactus sp. nov. This new knowledge has been used to compare the British ophthalmosaurs with American forms from rocks of the same age and it has been shown that Ophthalmosaurus, containing two species, is separate generically from Baptanodon which contains three.