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 Rh about 1210, embracing 1267 species, of which about 250 appeared under the joint authorship of Dr Greville and himself. These figures are in themselves sufficient evidence of the extent of his Pteridographic work.

It has been noted that the number of genera in the Genera Filicum was 135, maintained approximately according to the limitations of Presl in his Tentamen Pteridographiae: allowance has, however, to be made for 23 genera of Parkeriaceae, Schizaeaceae, Osmundaceae, Marattiaceae, Ophioglossaceae, and Lycopodiaceae, which were omitted in the Tentamen. But in the Synopsis Filicum there were only 75. It is true that the three genera of Lycopodiaceae were excluded also from the Synopsis, but still there is the wide discrepancy between 132 of Presl's genera as against 75 in Hooker's Synopsis. This at once indicates a salient feature of his method. He merged a large number of genera, ranking many of the smaller ones as subgenera under the more comprehensive headings. Doubtless the reasons for this were various. One was his mistrust of anatomical data, which it must be confessed Presl put too much in the fore-front. The very first sentence of the Tentamen runs thus "Vasa plantarum principale signum esse ex eo patet, quod exinde primaria divisio omnium plantarum exstitit." But occasionally Sir William explained his reason in a specific case. Thus in the question of Kunze's sub-genus Plagiogyria of the genus Lomaria, which Mettenius had raised to the dignity of a distinct genus, he explained his reasons for merging it into the genus Lomaria. Mettenius had laid stress upon various characters, but especially on the oblique annulus as distinctive. On this Hooker remarks "even should the capsules in all the species referred to Plagiogyria prove to be helicogyrate, yet the habit and sori are so entirely in accordance with true Lomaria that, unless the student has the opportunity of examining very perfect specimens, or unless he examines the structure of the annulus of the very minute capsules under the high power of the microscope, the genus cannot be identified. Kunze only proposed to form a group or section under the name of Plagiogyria, but even that would be found inconvenient to retain in a work whose main object is to assist the tyro in the verification of genera and