The Cycadeæ form a beautiful family of plants whose external habit resembles that of Palms, whilst their internal structure approximates in several essential characters to that of Coniferæ. In a third respect, (viz. the Gyrate Vernation, or mode in which the leaves are curled up at their points, within the buds,) they resemble Ferns; (See PI. 1. F. 33, 34, 35, and Pl. 58, 59.)
I shall select the family of Cycadeæ from the Fossil Flora of the Secondary period, and shall enter into some details respecting its organization, with a view of showing an example of the method of analysis, by which Geologists are enabled to arrive at information as to the structure and economy of extinct species of fossil vegetables, and of the importance of the .conclusions they are enabled to establish. Those who have attended to the recent progress of vegetable Physiology will duly appreciate the value of microscopic investigations, which enable us to identify the structure of vegetables of such remote antiquity, with that which prevails in the organization of living species.
The physiological discoveries that have lately been made with respect to living species of Cycadeæ, have shown them to occupy an intermediate place between Palms, Ferns, and Coniferæ, to each of which they bear certain
points of resemblance; and hence a peculiar interest attends
The most abundant deposit of fossil leaves of Cycadeæ in England, is in the Oolitic formation on the coast of Yorkshire, between Whitby and Scarborough, (See Philipps' Illustrations of the Geology of Yorkshire.) Leaves of this family occur also in the Oolitic slate of Stonesfield, Lindley and Hutton, Foss. Flora, Pl. 172, 175.
In Lindley and Hutton's Fossil Flora, Pl. 136, Figures are given of Cones which he refers to the genus Zamia, from the sandstone of the Wealden formation at Yaverland on the South coast of the I. of Wight.
M. Ad. Brongniart has established a new fossil genus Nilsonia, in the family of Cycadeæ, which occurs at Boer in Scania, in strata, either of the Wealden or Green-sand formation; and another genus, Pterophyllum, which is found from the New red sandstone upwards to the Wealden formation.
