Some Evolutionary Trends in the Pteridosperms

Abstract

A review of the Paleozoic and Mesozoic seed-ferns with particular emphasis on especially important subgroups. A brief discussion of the well known Calymmatotheca hoeninghausi is used as a central type and a review is given of Long''s recent descr. of the gametophyte of Lagenostoma ovoides. As representative of our knowledge of earlier seed-ferns a consideration is given of the Lower Carboniferous fossils from Scotland including Telangium affine, Tetrastichia bupatides, Megatheca thomasii, Calathospermum scoticum*, and Salpingostoma dasu. Evidence is presented which indicates that certain of these fossils may belong to the same sp. or a closely related group. The apparently heterosporous Archeopteris latifolia* from the Devonian of New York is considered and suggested as representing an early stage in the evolution of the pteridos-perm seed. Considerable space is devoted to the medullosas and associated microsporangiate and seed-bearing organs. To this is appended a short consideration of the Jurassic Beania gracilis and associated organs with suggestions concerning the origin of the cycadophytes from the medul-losan seed-ferns. Somewhat briefer treatment is given the Mesozoic corystosperms, Caytoniales, and Peltaspermaceae.