I. I ntroduction . T he Skiddaw Slates form the base of the great Silurian series of the north of England, and comprise a group of rocks entirely without admixture with interstratified igneous matter, attaining a thickness of about 7000 feet, as calculated by Prof. Harkness. Lithologically the series consists essentially of dark indurated shales, with a distinct cleavage, “having intercalated through them coarser strata, almost devoid of cleavage, possessing a flaggy nature, and affording fossils”‡. No distinct subdivisions can be drawn either on stratigraphical or on palæontological grounds, the beds being everywhere conformable and of nearly similar lithological aspect, while most of the characteristic fossils are found throughout. The upper beds of the series, however, are more shaly, softer, and darker in colour than the lower, whilst the peculiar genera Dichograpsus and Tetragrapsus do not seem to be represented in them, and there is a slight admixture of Upper-Llandeilo types. When the Skiddaw Slates were first studied by Professor Sedgwick, two Graptolites, and some obscure tracks (the so-called “ fucoids ”) constituted all their known organic remains; and the group was therefore considered by him the probable equivalent of the Longmynd or Cambrian Formation (see Synopsis Brit. Pal. Foss. Introduction, p. xxi). Since then, however, and mainly by the researches of Prof. Harkness, we have become acquainted with a large, if not very varied, falma from the Slates, and we are thereby enabled to refer the entire group with certainty to the age of the Lower Llandeilo rocks. At home the Skiddaw Slates have