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The author called attention to the various methods of classifying 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 consequence has been an enormous multiplication of genera; but, notwithstanding their number, the author found that if he adopted the methods of his predecessors he would have to establish additional ones for the reception of his new forms. Under these circumstances he decides that it will be better to include the entire series of these petioles, provisionally, under the common generic term ofRachiopteris . This plan dispenses with a number of meaningless genera, and is rendered additionally desirable by the circumstance that ah the petioles to which these numerous generic names have been applied belong to fronds which have already received other names, such asPecopteris, Sphenoteris , &amp;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 studying the modes in which secondary and tertiary vascular bundles were derived from the primary ones. This inquiry led him over the ground previously traversed by M. Trécul, and, so far as British ferns were concerned, by Mr. Church.

I. On the organization of the fossil plants of the coal-measures.—Part VI. Ferns