EXTINCT ISOSPOROUS EQUISETINÆ. In addition to several true species of fossilized _Equisetums_, the order of the CALAMITES, which no doubt is closely allied to the Equisetinæ, is also found in the fossil state. These were gigantic forms, attaining about twenty times the size of those of the present day, and stems of nearly 10–12 metres in height are known. They reached the culminating point of their development in the Carboniferous period, and died out towards the close of the Palæozoic. The stems had hollow internodes and alternating grooves, similar to their relatives of the present day. The leaves must either have been absent or very perishable, since they have not been identified with certainty. If the determinations of certain remains of cones which of late have been discovered are correct, they were heterosporous and had two kinds of sporangia as in the following sub-class. A cambium formation and an increase in thickness has been found in the stems. Their USES are very limited. A few species, such as _E. hiemale_ are used for polishing on account of the hard siliceous cell-walls of the epidermis, found in all species of _Equisetum_. Sub-Class 2. =Heterosporous Equisetinæ.= The two orders which come under this head are united by the characteristics, that the verticillate leaves are not united into sheaths (Fig. 226), and that between each whorl of fertile leaves there is also a whorl of barren ones. The fertile whorls in ANNULARIÆ are situated about midway between the barren ones (Fig. 227), but in ASTEROPHYLLITEÆ they occur immediately above a barren whorl (Fig. 228) and contain only half as many members as the latter. The lower whorls bear macrosporangia with one macrospore, the upper, microsporangia with many microspores. [Illustration: FIG. 226.--A. fragment of _Annularia_.] [Illustration: FIG. 227.--Fragment of _Annularia longifolia_, with sporangia; the leaves have partly fallen off: a barren whorls; _s_ fertile whorls.] [Illustration: FIG. 228.--Fragment of cone of _Asterophyllites_ (_Volkmannia elongata_): _a_ and _s_ as in Fig. 227.] The ANNULARIÆ were distichous (Fig. 226), and presumably floating plants. The ASTEROPHYLLITEÆ had verticellate branches. These also died out after the Carboniferous period, at the close of the Palæozoic. Class 3. =Lycopodinæ= (=Club-Mosses=). The characteristics of this class have been given on page 205. It consists of two sub-classes, one embracing isosporous, the other heterosporous forms. Sub-Class 1. =Lycopodieæ= (ISOSPOROUS Lycopodinæ). One kind of spore. Prothallium large, partly green. Leaves without ligule.