Two new ovules from the Fayetteville Formation blur the generic boundaries of Rhynchosperma and Stephanospermum and provide a possible link with Medullosa. The genus Rhynchosperma Taylor and Eggert was erected for permineralized seeds that are externally indistinguishable from the seed cast morphogenus Rhynchogonium. Described from four specimens from the upper Chesterian Fayetteville Formation of Arkansas, Rhynchosperma is radially symmetrical with a two layered integument. The apical portion of the integument is ribbed and non-vascularized; the nucellus is fused to the integument and apically differentiated into a dome shaped pollen receiving structure. The vascular system is poorly preserved and apparently restricted to the integument. Nothing is known about the plant that produced those ovules. In contrast, the trigonocarpalean genus Stephanospermum is widely regarded as belonging to medullosan seed ferns. As in all other trigonocarps, Stephanospermum is radially symetrical, with a stalked nucellus attached to the integument only at the base, and has a three parted integument. Seven species ranging from the Westphalian D to the Stephanian A-B boundary are placed in Stephanospermum, typically characterized by the presence of a sheath of tracheids in the nucellus and usually, a micropylar crown. The new Fayetteville specimens are similar to Rhynchosperma in external shape, two layered integument, number of ribs, and the presence of a dome shaped pollen receiving structure. However, they are more like Stephanospermum in vascular architecture and nucellar attachment in that the nucellus is vascularized by a sheath of tracheids, the integument is vascularized by discrete bundles, and the nucellus is attached to the integument at the base. A stalked nucellus has not been observed. The discovery of the Fayetteville specimens with characters of two unique Carboniferous ovules provides exciting insights into the evolution and phylogeny of upper Paleozoic seed plants.

Key words: integument, Medullosaceae, nucellus, Rhynchosperma, Stephanospermum