This paper is a contribution to the long-continued researches on the Dorset Lias by Dr W. D. Lang, F.R.S., who collected the described material. Some 700 mounted specimens from the Lower Lias (davoei zonc) of the Dorset coast have been studied. They are ascribed to six families, twenty genera, and fifty-five species; of these, eleven genera and forty-five species belong to the family Lagenidae. One genus and species, Carixia langi, and one other species, Lagena davoei, are described as new, and two new names are proposed to replace invalid names. There were studied for comparison certain of the few described English Lias faunas, some described French and German material, and a number of well-preserved but undescribed faunas from various Lias horizons and different English localities. This has yielded provisional evidence of the zonal ranges of the Dorset species. Some forms of Frondicularia were found to provide useful horizon markers. A tentative correlation is given of the zoning of the Lias by various authors of papers on Lias Foraminifera; there are notes on the horizons of some described Lias faunas, and a review of previous work on British Lias Foraminifera. Study of Jurassic Foraminifera has been comparatively neglected for many years, though there is a recent German revival. Some lengthy synonymies are therefore necessary. Certain less-known forms are discussed in detail; they were inadequately described and figured, so that they have hitherto been wrongly placed. It has thus been possible to rectify the systematic position or status of the genera Involutina, Problematina and Bullopora. The appearance in the Lias of Bolivina and Plectofrondicularia is demonstrated, genera usually stated not to occur in rocks of age earlier than the Cretaceous. Foraminifera thrived in the muddy Lias seas in whose clay deposits their shells are well preserved. There was a rapid evolution of new types, particularly of the predominating Lagenidae. In this family there appears to be wide variation within some of the groups, where neither 'species' nor even 'genera' are sharply defined. The bulk of the paper is taken up by the systematic description of the Foraminiferal fauna; all the recorded forms are figured. There is a reference list of some eighty-nine papers, mainly on Jurassic Foraminifera.