PETRIFIED PLANTS FROM THE UPPER MISSISSIPPIAN OF NORTH AMERICA. I: THE SEED RHYNCHOSPERMA GEN. N.

Petrified seeds of Upper Mississippian age (Chester Series) are described from sediments from northwestern Arkansas. Specimens of Rhynchosperma quinnii gen. et sp. n. are radially symmetrical and range in length from 1.2 to 2.2 cm and 1.0 to 1.3 cm at the greatest diameter. The seeds are ovate with a tapered apex having 8–10 integumentary ribs and a broadly rounded chalaza. The integument is two-parted, consisting of an outer presumably soft tissue with secretory canals and an inner layer constructed of longitudinally oriented thick-walled cells. Nucellus and integument are confluent, but they diverge near the seed midlevel with the free portion of the nucellus forming a dome-shaped pollen-receiving structure. Additional features such as vasculature and the structure of well-preserved megagametophytes are described. The petrified specimens are compared with taxa instituted for casts and impressions which include Rhynchogonium and Boroviczia. The ovulate versus cupulate nature of these genera which has remained uncertain until now appears to be settled in favor of an ovular identity.