Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/375

 Rh the recognition of similar structures in fossil plants, referable to a family whose characters are so remarkable.

The figure of a Cycas revoluta (Pl. 58,) represents the form and habit of plants belonging to this beautiful genus. In the magnificent crown of graceful foliage surrounding the summit of a simple cylindrical trunk, it resembles a Palm. The trunk in the genus Cycas, is usually long. That of C. circinalis rises to 30 feet. In the genus Zamia it is commonly short.

Our figure of a Zamia pungens, (Pl. 59,) shows the mode of inflorescence in this Genus, by a single cone, rising like a Pine Apple, deprived of its foliaceous top, from within the crown of leaves at the summit of the stem.

The trunk of the Cycadeæ has no true bark, but is surrounded by a dense case, composed of persistent scales which have formed the basis of fallen leaves; these, together with other abortive scales, constitute a compact covering that supplies the place of bark. (See Pl. 58 and 59.)

In the Geol. Trans. of London (vol. iv. part 1. New Series) I have published, in conjunction with Mr. De la Beche, an account of the circumstance sunder which silicified fossil trunks of Cycadeæ are found in the Isle of Portland, immediately above the surface of the Portland stone, and below the Purbeck stone. They are lodged in the same beds of black mould in which they grew, and are accompanied by prostrate trunks of large coniferous trees, converted to flint, and by stumps of these trees standing erect with their roots still fixed in their native soil. (See Pl. 57, Fig. 1.)