The genus Botrychium has few species, about half of which grow in North America. Botrychium Lunaria, or Moonwort, is the only species indigenous in Great Britain. This plant, which is from five to six inches high, has a rather long succulent stem, invested at the base by the dark-coloured membranous remains of withered leaves. It has one barren frond, the base of whose stem is a sheath, through which the tall, fertile frond rises. The barren frond has from three or four to seven pairs of opposite smooth pinnæ, of a lunate shape and glaucous green colour, overlapping each other; they are smooth, crenated on the margin, and occasionally lobed, which gives them a fan shape. The fertile frond is longer than the barren one, and ends in a panicle or thyrse. The sporangia, which are large and bivalved, are disposed in two regular series upon the divisions of the panicle, and directed towards the upper or inner face of the frond. This fructification bears a strong resemblance to that of Osmunda regalis, the Flowering fern. The stem of this plant has been assumed to be solid and branched, which is by no means the case; for, upon dissection, it is found to be hollow, and at its base the fronds of the following year may be detected more or less perfectly formed, and the rudimentary bud of the succeeding year within the latter; the position of the barren and fertile fronds being reversed in the successive developments. SECTION IX. EQUISETACEÆ, OR HORSETAILS.