In dealing with the fossil Equisetales, we will first consider the genera _Equisetites_, _Phyllotheca_ and _Schizoneura_, and afterwards describe the older and better known genera _Calamites_ and _Archaeocalamites_. A thoroughly satisfactory classification of the members of the Equisetales is practically impossible without more data than we at present possess. It has been the custom to include _Equisetites_, _Phyllotheca_ and _Schizoneura_ in the family Equisetaceae, and to refer _Calamites_ and _Archaeocalamites_ to the Calamarieae; such a division rests in part on assumption, and cannot be considered final. When we attempt to define the Equisetales and the two families Equisetaceae and Calamarieae, we find ourselves seriously hampered by lack of knowledge of certain important characters, which should be taken into account in framing diagnoses. There is little harm in retaining provisionally the two families already referred to, if we do not allow a purely arbitrary classification to prejudice our opinions as to the affinities of the several members of the Equisetales. The Equisetaceae might be defined as a family including plants which were usually herbaceous but in some cases arborescent, bearing verticils of leaves in the form of sheaths more or less deeply divided into segments or teeth. The strobili were isosporous and consisted of a central axis bearing verticils of distally expanded sporophylls with sporangia, as in _Equisetum_. The genus _Equisetites_ might be included in this family, but it must be admitted that we know next to nothing as to its anatomy, and we cannot be sure that the strobili were always isosporous. The genus _Schizoneura_ is too imperfectly known to be defined with any approach to completeness, or to be assigned to a family defined within certain prescribed limits. _Phyllotheca_ is another genus about which we possess but little satisfactory knowledge; we are still without evidence as to its structure, and the descriptions of the few strobili that are known are not consistent. Recent work points to a probability of _Phyllotheca_ being closely allied to _Annularia_, a genus included in the Calamarieae, and standing for a certain type of Calamitean foliage-shoots.