On the structures and affinities of Ankyropteris corrugata

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Abstract
Ankyropteris corrugata is a small zygopterid fern found in the calcareous nodules of the Lower Coal Measures of Lancashire and Yorkshire. It was first discovered by Williamson (46, b ), who included it in his comprehensive genus Rachiopteris , giving it the specific name corrugata on account of the fact that the external surface of the stem in his specimen was thrown into a series of irregularly transverse, round-topped ridges. The petioles were also described by him (46, c ) under the name Rachiopteris insignis ; Williamson recognised, however, that the petioles he had described as R. insignis were those of R. corugat , although he never actually published the fact (39). Rachiopteris corrugata was later included in Corda’s genus Zygopteris (15), in common with other palaeozoic ferns, the petioles of which were “characterised by a vascular strand having the form of an H in transverse section” (41). It owes its present generic name to Dr. Paul Bertrand, who, in his important memoir on the petiole-anatomy of the zygopterid ferns (4), has revived, in a limited and more rigidly defined sense, the name employed by Stenzel (42) in his sub-division of Corda’s genus.

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