A small permineralized osmundaceous stem has been identified from the Lower Cretaceous (Aptian), Upper Chickabally Member of the Budden Canyon Formation near Ono, California. The specimen, 8.5 cm long and 5.4 cm wide, was prepared using the cellulose acetate peel technique. The stem was erect, 13 X 11 mm in diam and surrounded by a mantle of stipular leaf bases and adventitious roots. The parenchymatous pith is surrounded by an ectophloic siphonostele with a xylem cylinder 9-13 tracheids thick and incomplete leaf gaps that extend 1/10 to 1/4 the distance through the xylem cylinder. A small, parenchymatous inner cortex and larger sclerenchymatous outer cortex contain from 65-79 leaf traces per cross section. Leaf traces are C-shaped, mesarch, with one protoxylem strand, sclerenchyma lining the concavity of the petiolar vascular strand. In the petiolar cortex, two large and several small sclerenchyma patches occur in the stipular wing and a heterogeneous sclerotic ring surrounds the trace. Numerous diarch, adventitious roots arise singly from the leaf traces as they depart from the xylem cylinder. The siphonostelic stem, lacking complete leaf gaps, having C-shaped leaf traces with one protoxylem cluster, petioles with sclerotic rings and stipular wings are characteristic of the genus Millerocaulis Erasmus ex Tidwell. Stem size, incipient leaf gap depth, number of cortical leaf traces, division of the protoxylem and sclerenchyma distribution distinguish this as a new species. The California specimen represents the youngest known species of Millerocaulis known and the first from the Northern Hemisphere.

Key words: Millerocaulis, Osmundaceae, Cretaceous, ferns, permineralized