There are three exposures of the horizon of the zone of O. pilula on the Dorset coast—at Ballard Head, Arish Mell, and Middle Bottom. Dr. Rowe's description of the Ballard Head section is so unattractive, and the section itself so inconvenient for access and study, that I put it aside to be attempted only if I had time to spare after working out the others, which has not yet happened. Of the other two, that at (or rather under) Middle Bottom is so much the better that I take it first.