In writing Part I. of the British Museum Catalogue of Fossil Reptilia, in which a large number of forms are known solely by disjecta membra, I was fully prepared to find that many of my determinations, which are frequently provisional, would subsequently need correction. It is, moreover, highly probable that there are really more species represented in the collection than I have indicated, as in some cases, in my desire to avoid the introduction of new specific or generic names which could not be fully substantiated, I have provisionally included under generic or specific headings specimens which may prove to indicate distinct forms.