Abstract T he author first describes the geological relations and distribution of the Skiddaw Slates, and notices their correspondence with the Quebec Group of Canada, and then gives a description of the Graptolites found in these rocks. The genera and their distinguishing characters are the following:— 1. Dichograpsus , Salter (3 species): possesses a frond, repeatedly dichotomous from a basal stipe into eight, sixteen, or more branches, each with a single row of cells, the lower part of the stipe being enveloped in a corneous cup. 2. Tetragrapsus , Salter (3 species): possesses a frond composed of four simple stipes, arising from a non-celluliferous funicle, which bifurcates at both ends. 3. Phyllograpsus , Hall (2 species): differs from the last in possessing a frond composed of four simple stipes united back to back by their solid axes. 4. Didymograpsus , M'Coy (7 species): the frond consists of two simple stipes springing from a mucronate radicle, which may be rudimentary or apparently absent. 5. Diplograpsus , M'Coy (4 species): two simple stipes, united by their solid axes into a celluliferous frond furnished with a radicle at the base. 6. Graptolites vel Graptolithus , Linn. (4 species): consists of a simple stipe, with a single row of cells on one side, and a small, generally curved, radicle at the base. 7. Pleurograpsus , Nicholson (1 species): celluliferous branches derived from a main celluliferous rhachis.