Arthropteris is a small genus of the Old World tropics (and southwards to New Zealand) the affinity of which is not clear. It shares with Oleandra and Elaphoglossum the character of outgrowths from the rhizome (phyllopodia) to which the stipes of fronds are jointed, and also has peltate rhizome-scales comparable with those of both genera. One group of species closely resembles Nephrolepis in shape and articulation of pinnae and in form and position of sori, another group has the frond-form and soral form of Thelypteris, and a third has sterile fronds so like those of Teratophyllum that they might be mistaken for that genus; the slender dorsiventral rhizome in all species is comparable with that of Teratophyllum, though the anatomy is different in detail. A remarkable feature, absent in all the other genera already mentioned, is the presence in Arthropteris of short multiseptate hairs essentially like those of Ctenitis and Tectaria; similar hairs occur in Davallodes (not in Davallia) and thus give a possible link with the Davallia group of genera (which all have dorsiventral rhizomes with articulated stipes, and peltate scales). The peculiar monotypic genus Psammisorus C. Chr. (Madagascar) shows features in common with Arthropteris and the Davallioid ferns. As regards spores, those of Nephrolepis, Oleandra, and Davallodes lack perispore, but a perispore is present in Arthropteris, Ctenitis, Teratophyllum, and Elaphoglossum. The original species of Arthropteris was the exindusiate A. tenella (Polypodium tenellum Forst.) and the genus was described by John Smith in J. D. Hooker’s Flora of New Zealand; Smith stated that there were also indusiate species which should be included in the genus. W. J. Hooker, however, did not recognize Arthropteris, and subsequently placed A. tenella in Polypodium, A. orientalis in Nephrodium, and A. palisotii in Nephrolepis. John Smith knew a large number of ferns as living plants under his care at Kew (a century ago he estimated that he had had a thousand species in cultivation, though doubtless not all at one time) and in distinguishing genera he took into consideration characters, noted from the living plants, which Hooker was not prepared to accept as significant for the purpose. I agree with Smith in accepting Arthropteris as a natural genus, and believe that its peculiar combination of characters may give significant hints as to inter-relationships among various groups of ferns, but it is not yet clear to me how such inter-relationships may best be formally expressed in a scheme of classification. Arthropteris is usually placed near Nephrolepis, but I do not think Nephrolepis is its nearest relative.