This plant is a true equisetum, differing chiefly from existing species in its gigantic size and arborescent character. The sheaths surrounding the stem, and the verticillate linear leaves, are preserved in some examples: and in all, the furrows left by the imprints of the sheaths are more or less strongly impressed. The stem is not channelled throughout, as in _Calamites_, the carboniferous plant whose stems at first sight might be mistaken for those of Equisetites, but which are entirely distinct, as will be explained hereafter. The Equisetites columnaris is peculiar to the Oolite; it does not occur in the coal-measures. Specimens have been discovered which indicate a height of twenty feet, and a diameter of several inches.[76] [76] See Hist. Vég. Fossiles, p. 115. A small species of Equisetum (_Eq. Brodiei_[77]) occurs in the insectiferous limestone of the lower Lias, at Strensham, Worcestershire, associated with the foliage of fresh-water endogenous plants resembling the _Potamogeton_, or pond-weed, and of supposed dicotyledonous vegetables. [77] Prof. Buckman, in Geol. Journal, vol. vi. p. 413. [Illustration: Lign. 14. Calamites decoratus. 1/3 _nat._ (Ad. Brongniart. Pl. 14.) _Coal Formation. Yorkshire._ Fig. 1.--Part of a stem, showing the tubercles for the attachment of leaves. 2.--A portion of the same on a larger scale. ] [Sidenote: CALAMITES.] Calamites. _Lign. 14, 15_.--Stem articulated, regularly furrowed longitudinally, the articulations naked, or studded with tubercles. The plants of this genus were supposed to be related to the marestail, but to differ in the absence of the encircling sheaths, and in being uniformly striated; but an examination of specimens in a better state of preservation than those previously known, shows their affinity to the gymnosperms. Some of the species are of a gigantic size, being from one to three feet in diameter, and from thirty to forty feet in height. Calamites abound in the coal formation, and must have constituted an important feature in the forests of the carboniferous period; they occur also in more ancient deposits, and some species belong to the earliest terrestrial Flora of which any vestiges are known. In most instances when specimens are found lying in the same plane with the strata, they are pressed flat, but those occurring in a vertical position retain their natural cylindrical form. An outer crust or cylinder of coal generally invests the stem, but traces of the internal structure are rarely preserved. The Calamite consists of a large central column of tissue, surrounded by a ligneous cylinder. The central part has in most instances perished after the death of the plant, and the cavity thus left been filled up with mineral matter. As the hollow ligneous zone is almost always carbonized, and very friable, it is seldom attached to the cast, and consequently the surface of the latter is generally jointed and ribbed.