At the request of Professor J. W. Gregory, F.R.S., of Glasgow University, the following report has been prepared dealing with some microscopical organisms entering into the composition of certain limestones from Central New Guinea. The material studied, comprising Foraminifera and Nullipore (Lithothamnium) remains, has been obtained from some rolled limestone pebbles which were collected in river-beds of the upper reaches of the Fly River by the Right Hon. Sir William Macgregor, G.C.M.G., during an expedition carried out in the years 1889 and 1890. Being desirous that these limestones should be scientifically examined, Sir William submitted them to the Geological Department of the University Glasgow in the hope that Professor Gregory and his staff might issue a statement as to their structural and geological value. In accordance with this request, therefore, the corals were first examined and described in a joint paper by Professor Gregory and Miss J. B. Trench, while some remarks on the Foraminifera and associated structures have been postponed for the present occasion. At the time when the corals were in course of description, the writer was appealed to for an opinion as to the geological age of one of the pebbles (No. 1), exhibiting corals and some smaller organisms, although to give aid in this direction it was necessary to prepare microscopical sections of the limestone for examination by transmitted light.