Arachnoxylon from the Middle Devonian of southwestern Virginia

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Abstract
Eight specimens from the Middle Devonian Purcell Member of the Millboro Shale Formation in southwestern Virginia conform to a recent definition of the previously monotypic genus Arachnoxylon. The new material provides supplementary information on several tissue regions of the main axis, including evidence on variability in the structure of the protoxylem, and on departure of the large, or "major" traces. In addition, evidence is provided here for the first time on a discrete region, immediately adjacent to the primary xylem, containing probable conducting elements of the primary phloem, on the structure of the epidermis, and on the presence of multicellular superficial emergences. Although clearly assignable to Arachnoxylon, all but one of our specimens vary significantly from previous specimens in the genus in several features which may be interpreted as relating directly or indirectly to size. It is possible that all specimens in Arachnoxylon represent portions of a single kind of plant. However, organic connection between the large and small forms has not been demonstrated. We elect, therefore, to establish A. minor sp. n. for the smaller specimens.