There are four genera of this order, very widely dispersed, most abundant and larger in the hot moist parts of India and the Indian islands, but large tracts are covered with them within the arctic circles and in temperate latitudes. There are 200 species of the genus Lycopodium, or Wolfs-claw, and of these, six only are indigenous in Britain; they grow in very exposed situations, as in the case of L. clavatum, which lives on upland heaths and pastures, and which has a procumbent stem, creeping for many feet or yards, sending out branches in all directions, with a pair of cones at their extremities, and strong roots at intervals to fix it to the ground. Lycopodium inundatum forms large patches on the marshes in the south of England. New Zealand has many more species of this genus than Britain, and some of the noblest specimens. The dried spores of the L. clavatum are so inflammable, that they have been used on the stage to produce the effect of lightning. Some Club Mosses yield a blue dye, a colour which is now obtained of a better quality from coal tar. Others possess cathartic properties, but although they have been used as medicine, they are very dangerous from the violence of their effects. The genus Selaginella has a moss-like habit. The stem is generally creeping, and flat-looking from being clothed with distichous leaves; but sometimes it is tall and erect, having feathery branches clad with leaves. The sporangia are sacs, with two or three valves containing large spores and sub-globose antheridia, containing orange-coloured or minute scarlet bodies, ultimately developed into spermatozoids. The species are very numerous and greatly varied, and mostly tropical. From a comparison of the structure of the stems of the fossil plants, Sigillaria and Lepidodendron, Dr. Hooker concludes that they are highly developed Lycopods, approaching closely in structure to the highest class of plants. The stems of the former differ chiefly in size from those of the Lycopodium, and the cones of the Lepidostrobus differ in their greater development and in the thickness of their scales from those of the Conifers. The spores of the Lepidostrobus ornatus too are sphericotetrahedral, like those of the Lycopodium. But inferior as the existing Lycopods are to their fossil ancestors, their analogy to the Conifers gives them a more exalted position in the vegetable world than their tall and graceful allies, ‘the tree ferns.’