522 Royal Society : — PROCEEDINGS OF LEARNED SOCIETIES. ROYAL SOCIETY. March 26, 1874. — Joseph Dalton Hooker, C.B,, President, in the Chair. " On the Organization of the Fossil Plants of the Coal-measures. —Part VI. Ferns." By W. C. Williamson, F.E.S., Professor of Natural History in Owens College, Manchester. The author called attention to the various methods of classify-ing the fern-stems and petioles of the Coal-measures adopted by Cotta, Corda, Brongniart, and others, and to the difficulties which attend those methods. Some of those difficulties had been already felt and partially removed by M. Brongniart. All the generic distinctions hitherto adopted were based upon variations in the form, number, and arrangement of the vascular bundles. These elements vary so much, not only in different species of the same genus, but in different parts of the same petiole, as to make them most untrustworthy guides to generic distinctions. The conse-quence has been an enormous multiplication of genera ; but, notwithstanding their number, the author found that if he adopted •the metliods of his predecessors he would have to estabhsh addi-tional ones for the reception of his new fonns. Under these circumstances he decides that it wWl be better to include the entire series of these petioles, provisionally, under the common generic term of Rachiopteris. This plan dispenses ■«'ith a number of meaningless genera, and is rendered additionally desirable by the circumstance that all the petioles to which these numerous generic names have been applied belong to fronds which have already received other names, such eksPecoj)ttris, S2)henopteris, &c. ; only the structure of fronds found in the shales, and their respective petioles of which we have ascertained the structure, have not yet been correlated. As a preparation for the present investigation, the author made an extensive series of researches amongst recent British and foreign fern-stems and petioles, with the object of ascertaining not only the modifications in their arrangements in different parts of the same plant, but especially of studving the modes in which se-condary and tertiary vascular bimdles were derived from the primary ones. This inquiry led him over the ground preA'iously traversed by M. Trccul and, so far as British ferns were concerned, by Mr. Church. The most common general forms exhibited by transverse sections of these bundles in recent petioles may be represented by the letters H, T, U, and X. As a general rule, the secondary bundlevS are given oft' from that part of the primary one which happens to be neai-est to the secondary rachis to be suppUed. Thus in some ca^es the upper arms of the X will merely be prolonged and their '