To determine the botanical relations of fossil leaves and stems, reference must be had to works expressly devoted to the subject; namely, the "_British Fossil Flora_," by Dr. Lindley and Mr. Hutton, and the "_Histoire des Végétaux Fossiles_," by M. Adolphe Brongniart. The classification of the last-named eminent botanist is here adopted, as the most easy of application. With regard to the nomenclature, it may be necessary to remark, that when a fossil plant undoubtedly belongs to a recent genus, the usual botanical name is employed: for example, _Equisetum Lyellii_; when the fossil does not possess all the generic characters, yet is evidently allied to a recent genus, the term _ites_ (from λιθος, _lithos_, stone), is added--as _Equisetites_, _Palmacites_, &c.; and this termination is invariably adopted by some authors. When the fossil plant differs altogether from any known type, it is distinguished by some arbitrary generic name, as _Bucklandia_, _Sigillaria_, &c. There are also a few provisional genera for the reception of fossil leaves, fruits, and stems, whose characters and relations are but imperfectly known; as Carpolithes, Endogenites, &c. Upon these principles the present arrangement has been founded: the progress of discovery will, of course, be continually adding to the list, and the classification require to be modified. [Sidenote: FOSSIL CRYPTOGAMIA.] The following account of the principal types of the ancient floras whose relics are preserved in the mineral kingdom, though commencing with those of the most simple structure, the _Cryptogamia_, and advancing to the higher orders, is not strictly botanical; for it was found convenient, in some instances, to notice certain species and genera of different orders under the same head, from their occurrence in the same geological formations. It is estimated that not more than two thousand species of plants have been discovered in a fossil state, while the known recent species amount to upwards of eighty thousand. Cellular Cryptogamia; Algæ.--The plants designated by botanists _Algæ_, and commonly known as sea-weeds, lavers, and fresh-water mosses, are of the most simple structure--mere aggregations of cells--but present innumerable varieties of form and magnitude: many species are mere vesicles of such minuteness as to be invisible to the unassisted eye, except accumulated in countless myriads, when they appear as a green, purple, or reddish, slime in the water; or as a film on wood or stone, or on the ground, in damp situations; while others are tough branched marine plants, many fathoms in length.