The fossil genera have been established by M. Ad. Brongniart, from the form of the leaves and the characters of their venation; that is, the distribution of the vessels. In the following descriptions some botanical phrases are necessarily employed; a few terms of frequent occurrence are explained in _Lign. 16_. Pachypteris[81] (_thick-fern_). _Lign. 17._--In this genus from the lower Oolite, the fronds are pinnated, or bipinnated, the leaflets entire, without visible veins, having but a single midrib, and contracted at the base. The absence of veins, and the leaflets not being lobed, are the essential generic distinctions. [81] The names of the genera are derived from _pteris_, fern, to which prefixed a term indicative of the peculiar characters. [Illustration: Lign. 17. Pachypteris lanceolata. _Inferior Oolite. Whitby._] [Illustration: Lign. 18. Sphenopteris elegans _Coal-shale, Waldenburg, Silesia._] [Sidenote: FOSSIL FERNS. SPHENOPTERIS.] Sphenopteris (_wedge-leaf_). _Lign. 18_.--The leaves are twice or thrice pinnated, the leaflets wedge-shaped, contracted or narrowest at their base, and more or less deeply lobed: the lobes divergent and palmated: the veins radiating from the base. The ferns of this genus are extremely elegant, and comprise upwards of forty species. A beautiful Sphenopteris (_S. affinis_, _Wond._ p. 716,) occurs abundantly in the fresh-water carboniferous strata at Burdie House, near Edinburgh;[82] another elegant form, in coal-shale, is represented in _Lign. 18_. [82] See Dr. Hibbert's Memoir on the Strata and Fossils of Burdie House. 4to. 1835. [Illustration: Lign. 19. Sphenopteris nephrocarpa. _Inferior Oolite, Scarborough._ A magnified vein of a leaflet, showing the fructification at the extremities of the lobes, × two diameters.] It is so rarely that the fructification of any species of Sphenopteris is preserved in a fossil state that I am induced to figure a leaflet of a remarkable plant, of this genus from the fluvio-marine oolitic deposits of Scarborough. _Lign. 19_ is copied from the lithograph accompanying a notice of some rare plants from that locality, by the eminent botanist, C. J. F. Bunbury, Esq.[83] [83] Geol. Journal, vol. vii. p. 179, pl. xii. This fossil fern closely resembles certain species of _Dicksonia_ (natives of New Granada). Each segment of the leaflet or pinnate is dilated at the apex into a reniform _indusium_; no capsules are visible, the fructification being, probably, in a young state. [Illustration: Lign. 20. Sphenopteris Mantelli; _nat._ _Wealden, Tilgate Forest._]