An excellent work ("_Dendrolithen_") on these fossils, in which thirty species are described, has been published at Dresden by M. Cotta; who arranges them under the genus _Psaronius_ or _Psarolites_. The stem is composed of two distinct parts; an outer zone, consisting of a great number of nearly cylindrical bundles of vessels, supposed to have been roots which proceeded from the stem near its base; and an inner part or axis. In the outer portion, the fossil _air-roots_ have a vascular tissue, but there is often a delicate cellular tissue interposed. In the axis the vessels form zigzag or wavy bands, resembling those of ferns.[90] These flexuous and vermiform bands are entirely composed of barred or scalariform vessels, similar to those of ferns and club-mosses. The Psarolites are therefore considered by M. Ad. Brongniart to be the bases of the trunks of lycopodiaceous trees, while M. Cotta and other botanists regard them as true arborescent ferns.[91] [90] Pict. Atlas, pl. viii. [91] See M. Brongniart's "_Tableau des Genres de Végét. Foss._" p. 44. Dr. Buckland has discovered in the New Bed sandstone formation at Allesley, near Coventry, silicified trunks of coniferous trees, and it is not improbable that further research in that locality may bring to light fern-stems like those of Chemnitz.[92] Dr. Lloyd, of Warwick, has recently obtained leaves of several coniferæ from the same locality.[93] [92] Vide Geol. Proc. vol. ii. p. 438. [93] Geol. Society, June 1852. Dr. Lloyd's specimens are probably referable to the genus _Walchia_: see _Lign. 60._ [Sidenote: SIGILLARLÆ AND STIGMARIÆ.] Sigillarlæ and Stigmariæ.