Abstract
Stenomyelon bifasciculare nov. sp. is described from the Tournaisian of the Montagne Noire. Represented by six fragments of stems measuring 25 to 40 mm in diameter, these fossil axes bear swollen petiole bases separated by short internodes. Anatomically, they are characterized by a mixed stele with eccentrically mesarch protoxylem poles, secondary xylem with tracheids pitted on all walls, petiolar traces of Kalymma type, and few sclerotic nests in the cortex. The divergence of a pair of leaf traces from each rib of the stele has given the taxon its specific epithet. This feature is new for the genus. Stenomyelon bifasciculare is more distinctive than the Scottish and American taxa described to date. Except for Stenomyelon heterangioides, in which this character is doubtful, no sympodia from which leaf traces diverge are recognized in any Stenomyelon, including Stenomyelon bifasciculare; protoxylem maturation has no cauline expression. This character is suggested as the most important one to define the genus.
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