In accordance with M. Brongniart's generalisation, that the Palæozoic period is, botanically speaking, the "Age of Acrogens," we find the Carboniferous plants to be still mainly referable to the Flowerless or "Cryptogamous" division of the vegetable kingdom. The flowering or "Phanerogamous" plants, which form the bulk of our existing vegetation, are hardly known, with certainty, to have existed at all in the Carboniferous era, except as represented by trees related to the existing Pines and Firs, and possibly by the Cycads or "false palms."[18] Amongst the "Cryptogams," there is no more striking or beautiful group of Carboniferous plants than the _Ferns_. Remains of these are found all through the Carboniferous, but in exceptional numbers in the Coal-measures, and include both herbaceous forms like the majority of existing species, and arborescent forms resembling the living Tree-ferns of New Zealand. Amongst the latter, together with some new types, are examples of the genera _Psaronius_ and _Caulopteris_, both of which date from the Devonian. The simply herbaceous ferns are extremely numerous, and belong to such widely-distributed and largely-represented genera as _Neuropteris, Odontopteris_ (fig. 108), _Alethopteris, Pecopteris, Sphenopteris, Hymenophyllites_, &c. [Footnote 18: Whilst the vegetation of the Coal-period was mainly a terrestrial one, aquatic plants are not unknown. Sea-weeds (such as the _Spirophyton cauda-Galli_) are common in some of the marine strata; whilst coal, according to the researches of the Abbé Castracane, is asserted commonly to contain the siliceous envelopes of Diatoms.]