I. Introduction The general features and fossils of the Wenlock Limestone have been described by several geologists, but I am not aware of any endeavor to make a systematic examination of the microscopic structure. The value of such work has been greatly underestimated; for investigations which I have made show that, although much information as to the formation and structure of the rock may be gained by observations made without the assistance of a microscope, yet the use of this instrument brings into view important facts which are not perceived by the unassisted eye. In this paper I do not attempt an exhaustive enquiry; to do this it would be necessary to make several visits to the localities from which specimens for examination were collected, and this I have been able to accomplish only in the case of the Wenlock Limestones exposed within a convenient distance from Cheltenham. I have therefore selected what I regard as typical sections of the limestone in the neighbourhood of May Hill in Gloucestershire, and in the adjoining portion of the county of Hereford. The procedure has been to take samples from several beds in each exposure of the rock, and to grind down the usual thin sections for examination under a microscope. These I prepare myself, and I mention the fact because it seems to me important for this reason: very many slides require to be made for a paper like the present one, and each must be adjusted, with regard to actual thinness,