Trunks and Stems of Cycadaceæ.—In this section I shall notice the fossil plants which occur so abundantly in the fresh-water deposits that overlie the marine oolitic limestone of the Isle of Portland, and which must be familiar to my readers, from the graphic account of the circumstances under which they occur, by Mr. Webster, and subsequently by Dr. Buckland, and Sir H. De la Beche. In my Wonders of Geology, p. 387, and Geol. Isle of Wight, p. 395, the geological phenomena of that most interesting locality, the Isle of Portland, are so fully described, that it will not be necessary to dwell upon them; the structure and affinities of the fossil vegetables are the especial objects of our present inquiry.
MANTELLIA NIDIFORMIS.
Lign. 50. Silicified Trunk of Mantellia nidiformis (Brongn.); 1/4 nat.
(Cycadites megalophyllus. Bd.) Wealden. Isle of Portland.
a. Central mass of cellular tissue, b. Circle of ligneous plates,
c. Zone of cellular tissue, d. False-bark.
Mantellia. Lign. [50], [51.]—The fossil Cycadeæ of the Isle of Portland were first described botanically by Dr. Buckland, (Geol. Trans, vol. ii. 2d Series,) under the name of Cycadeoidea; of which Memoir the account in Bd. p. 404, is an abstract. M. Ad. Brongniart, considering these plants as a peculiar type, referred them to a new genus, which he did me the honour to name Mantellia (Prod. Veg. Foss.). These stems or trunks are from one to two feet in height; the circumference of the largest not exceeding three feet. The stem is subcylindrical, and the external surface covered with the rhomboidal scars formed by the attachment of the leaf-stalks, and which are widest in their transverse diameter.
There are two species, which are readily distinguished by the form of the stems, and the difference in the size of the cicatrices left by the petioles.
The most common kind is short, and spheroidal, and the leaf-scars are relatively large; its shape has caused it to be named "Crows' nest," by the quarrymen, who believe these plants to be nests that were built by crows in the trees of the petrified forest with which they are imbedded. The specific name (nidiformis) adopted by M. Brongniart, expresses this popular notion.
Lign. 50. represents a fine example from the Portland Dirt-bed, which exhibits a structure altogether similar to that which characterizes the stems of recent cycadeous plants; namely, (a) a central mass of cellular tissue surrounded by circles of laminated ligneous rays or plates (b); then a zone of cellular tissue (c), and an external cylinder of false-bark (d). The mode of increase by buds, from germs in the axillæ of the petioles, as in the living plants, is also distinctly seen.