Ichthyosaurus and Stenopterygius are the two best-known ichthyosaurs, represented by numerous specimens and having a long history of paleontological study. Despite such advantages, it is shown here that a major error in skull roof reconstruction, which dates back to the time of Richard Owen, still dominates the literature. Contrary to the conventional wisdom, the two genera share a unique feature, namely the separation of the frontal and postfrontal by the prefrontal. This hitherto ignored feature is phylogenetically important for clarifying the validity, synapomorphy, and membership of the Thunnosauria, a clade defined by the two genera. The feature is also useful in basic taxonomy. Many isolated skull specimens from the lower Lias of England are identified as Ichthyosaurus. Some of the skulls lack the prefrontal feature and differ from Ichthyosaurus in several other respects, suggesting that they represent at least one undescribed genus in the Lower Lias of England. This coincides with the previously reported presence of forefin specimens that clearly do not belong to known genera. Lower Liassic ichthyosaurs were more diverse than previously thought.