It was earlier placed under Zamia gigas by Williamson (1868). Later it was named in honour of its discoverer. Williamsonia sewardiana is the best known species described by late Professor Birbal Sahni from Rajmahal Hills, Bihar (India). Gupta (1953) later described another species from Rajmahal Hills and named it after Professor sahni.
Cycadeoidea ( = Bennettitis) is a fossil that includes many species. Many petrifications have been found of which some are complete plants. This fossil has been found in Jurassic and cretaceous strata of Dakota, America.
Lyginopteris is a palaeozoic pteridosperm. It is a stem genus of Calymatotheca Hoeninghausi. The descriptions of Lyginopteris Oldhamia are available. The genus was described under Dadoxylon oldhamium by Binney in 1886.
Genus Calamites appeared in Upper Devonian, and was most abundant during Carboniferous and became extinct early in the Triassic. The plant was 20 to 30 meters tall tree.
Lepidodendron with over 100 species, is the largest known genus. It is found in the shales and sandstones of carboniferous coal bearing formations.
HORNEOPHYTON LIGNIERI with one species H. lignieri was found with Rhynia in the Rhynichert bed of Aberdeenshire, Scotland. It was previously named as Homeo.
Rhynia is a fossil member (not found living in the present age), discovered from Rhynichert Beds (or upper Devonian era) in Aberdeenshire of Scotland, by Kidston and Lang in 1917.