ABSTRACT─Three isolated, partial skulls from historic collections, previously identified as Ichthyosaurus communis are herein assigned to Protoichthyosaurus prostaxalis. A fourth, nearly complete skull, NHMUK R1164 is referred to Protoichthyosaurus applebyi, only the second known specimen of the species. It provides additional information on the posterior portion of the skull and mandible. The diagnosis of P. applebyi is emended to include a postorbital that is dorsoventrally long but anteroposteriorly narrow relative to its length. It separates most of the jugal dorsal ramus from the orbit margin and makes up much more than half of the orbit posterior margin. The genus Protoichthyosaurus can be most easily distinguished from its sister taxon, Ichthyosaurus, by the forefin morphology, with three primary digits (II-IV) in the former and four primary digits (II-V) in the latter. In addition, the pineal in Protoichthyosaurus is bordered posteriorly by the parietals rather than being entirely enclosed by the frontals as in Ichthyosaurus. Many skull features, although not unique to Protoichthyosaurus, can together distinguish it from Ichthyosaurus. These include: a low-crowned skull with a long, slender rostrum; a large, posteriorly high, triangular external naris; an asymmetric maxilla with a long anterior process; a dorsoventrally long prefrontal anterior process that separates the dorsal process of the lacrimal from the orbit margin; and tooth roots with deep grooves that extend to the base of the crown. However, these characters vary among individuals and are often difficult to assess because of orientation, completeness, or preservation. Characters that distinguish individual species of Protoichthyosaurus from individual species of Ichthyosaurus are less subjective and often more easily evaluated.