Vascular, or Acrogenous Cryptogamia.--These plants, as the name implies, possess a more complicated structure than the preceding, having vascular tissue as varied as in the phanerogamia. Equisetaceæ.--The common species of Equisetum, or Marestail, is a plant that grows in marshy tracts, and on the banks of ditches and rivers; it has a jointed stalk, garnished with elegant sheaths which embrace the stem, and verticillate linear leaves: it attains a height of two feet, and is half an inch in diameter. In the fossil state there are many plants allied to the Equisetum, but only a few that are generically the same. [Illustration: Lign. 12. Equisetum Lyellii. _Wealden. Pounceford._ _nat._ Fig. 1.--A stem, having two sheaths, and a bud at the lowermost joint. 2.--Stem of a young plant, with sheaths, preserved in pyrites. 3.--Stem, with the cryptogamous head or upper end. ] [Sidenote: EQUISETITES.] Equisetum Lyellii, _Lign. 12_.--A species which I discovered in Wealden limestone, at Pounceford (_Geol. S. E._ p. 245), must have closely resembled the _Equisetum fluviatile_: it has an articulated cylindrical stem, and regularly dentated sheaths, embracing the stem at the joints. A transverse slice of the stem exhibits under the microscope a cellular structure filled with calc-spar, and forms a beautiful object when viewed with the polarizing apparatus. This plant occurs in many localities of the Wealden in Sussex and Kent; from the railway cuttings near Tonbridge, I collected several fine specimens; it is met with also in the cliffs near Hastings. Equisetites columnaris. _Lign. 13_.--A gigantic species of Equisetum abounds in the strata of the lower division of the Oolitic or Jurassic formation of Yorkshire, and many fine specimens have been collected, especially from the vicinity of Whitby. In the sandstone of the Inferior Oolite of the Cleveland Hills, Yorkshire, numerous stems of this colossal marestail have been observed standing erect, as if occupying the position in which they grew; the same fact was also discovered at Carlton Bank, near Stokesly, forty miles from the coast. In both localities fossil shells of fresh-water mussels (_Uniones_) were associated with the vegetable remains. [Illustration: Lign. 13. Equisetites columnaris. (Ad. Brongn. Pl. 13.) _Lower Oolite. Whitby._ Fig. 1.--Portion of a stem, showing two articulations, and an intermediate constriction. 1/3 _nat._ 2.--A few of the denticulations produced by the sheath, _nat._ ]