THE well-known horse-tails of the present day, common in marshes and shallow water, are herbaceous plants which at most grow to a height of about six feet, and are usually smaller. Their relatives in the Carboniferous period were very similarly organised, and probably had quite a similar appearance, but were woody trees often of considerable height. The delicate branches bore whorls of linearand the cones, borne on the ends of slender branches, were sometimes six inches long. Various detached remains of the tree horse-tails or Calamites, particularly what are known as pith-casts, are exceedingly abundant as fossils in the coal measures. The were hollow, and became filled with sand or mud when they fell into the swamps, streams, or pools where they grew; when the decayed and the sediment hardened a cast of the hollow pith was formed, showing the transverse joints of the original stem.
The cones were more elaborate than in the living horsetails, and those of the Upper Carboniferous type were rather more highly organised than the Upper Devonian and Lower Carboniferous forms. It would be a mistake to suppose that the living horse-tails are the direct, simplified, and as it were degenerate, descendants of the ancient ones. The two families are closely related, but there is no evidence that the older group is ancestral. There is indeed some evidence that herbaceous horse-tails may also have been in existence in the Coal Measure age, but the specialised giant forms that dominated the swamps of those times seem to have been completely extinguished by the close of the Palaeozoic era.