Measurements of the particle size of some Ordovician limestones from Öland and the Siljan district of Sweden have been made on thin slides by means of a micrometer according to the method of Delesse-Rosiwal. The percentage of fossil fragments (other particles of sand size being very rare in the limestones studied) with a maximum length on the thin slide of more than 0,1 mm has been determined, distinguishing thereby between a »sand fraction» with particle size greater than 0,1 mm and a »groundmass» of finer grains including the secondarily cementated pore spaces. The procedure described allows a rough comparison of different kinds of sedimentary limestones according to their general coarseness. The terms of Grabau (1901), calcarenite and calcilutite, originally intended only for elastic limestones, are recommended for use as strictly granulometrical terms for all types of sedimentary limestones in which the original form of the particles has not been destroyed by recrystallization. It seems preferable to draw the limit between calcarenite and calcilutite at about 20% of the »sand fraction» in limestone. — The limestones studied included various types of coarseness.