I. Introduction. In Somerset the Rhætic rocks have a greater superficial extent than in any other county. It was anticipated that their investigation would prove of exceptional interest for several reasons: notably because— (a) It has long been known that the Series attains an unusual thickness in places in this county, while it also frequently assumes a littoral facies when in contact with the Palæozoic rocks that formed land-areas during its time of deposition; (b) The discovery in the top-portion of the ‘Grey Marls’ of Watchet of Microlestes teeth, associated with characteristic Rhætic fossils, has long been an important fact in the discussion as to whether the ‘Grey and Tea-green Marls’ (auctt.) should be classed with the Rhætic or with the Keuper; (c) The Pteria-contorta Shales, with their associated hard beds, have yielded a rich assemblage of invertebrate fossils; and (d) The White Lias is excellently developed, and its relationship to the Lower Lias can be studied in many sections. In order, however, that students of the Series should have at their disposal easily-comprehended facts concerning the detailed stratigraphy, as also a knowledge of the position of certain strata bearing well-known names, and the horizons whence many of the well-preserved specimens now housed in our museums came, much detailed work was necessary. The main points that required to be settled were:— (a) The stratigraphical position of the marls that had yielded the Microlestes teeth and other characteristic Rhætic fossils; (β) The true position of the ‘Wedmore Stone,’ Moore's ‘Flinty Bed,’