Because fossils of complete fern plants have not yet been found, separate names have been adopted for detached leaves, stems, and other parts. For example, the fossil stems of some Pennsylvanian ferns found in Illinois have been referred to two genera, _Megaphyton_, whose leaf attachment scars are arranged in two vertical rows, one on either side of the stem, and _Caulopteris_, whose leaf scars are arranged in a steep spiral that becomes progressively flatter upward until near the top they appear to be whorled. When the stem is a petrifaction, with internal structures preserved, it is called _Psaronius_. The fronds are referred to a number of genera, but those most commonly found in Illinois are _Pecopteris_, _Asterotheca_, and _Ptychocarpus_. [Illustration: Venation of seed fern leaflets] _Pecopteris_ _Asterotheca_ _Ptychocarpus_ SEED FERNS (Plate 4) Seed ferns resembled true ferns in general, but they produced seeds, borne on modified leaves. Where spore sacs and seeds are absent, the leaves of seed ferns are difficult to distinguish from those of spore ferns, although individual seed fern leaflets, called pinnae, are somewhat larger. [Illustration: _Medullosa_ Reconstruction and original drawing by Wilson N. Stewart] Seed ferns included vinelike plants in the undergrowth and trees such as _Medullosa_. Some tree genera were very tall, with trunks more than two feet in diameter. Unlike the true ferns, still living today, seed ferns declined steadily after the close of the Pennsylvanian Period and finally became extinct during Jurassic time. During Pennsylvanian time, however, they were much more numerous and varied than true ferns. [Illustration: Venation of seed fern leaflets] _Alethopteris_ _Odontopteris_ _Mariopteris_ _Neuropteris_ _Linopteris_ Most of the common seed ferns found as fossils in Illinois can be referred to the following leaf genera: _Alethopteris_, _Neuropteris_, _Odontopteris_, _Linopteris_, _Mariopteris_ (which may be a true fern), _Cyclopteris_, and _Spiropteris_. _Cyclopteris_ includes circular leaves that occurred at the base of leaves referable to _Neuropteris_. _Spiropteris_ includes young leaves that had not yet uncoiled and may belong to either true ferns or seed ferns. CORDAITES (Plates 1 and 2) [Illustration: Reconstruction of _Cordaites_ (after Hirmer)] Cordaitean trees, forerunners of modern conifers such as pine and spruce, were important during the Pennsylvanian Period for they were distributed throughout the world. These trees, among the tallest plants of the time, sometimes grew more than 100 feet high.