256 PTERIDOPHYTA. [CH. 



Equisetum characters. The sheath-like form of the leaves is not 

 necessarily peculiar to the Equisetaceae, and we have evidence 

 that leaf-sheaths occurred on the nodes of Calamitean plants. 

 In Archaeocalamites the leaves possess characteristic features, 

 and can hardly be said to agree more closely with those of 

 Calamites than with the leaves of Phyllotheca or Sphenophi/llum, 

 a genus belonging to another class of Pteridophytes. 



On the whole, then, without discussing further the possi- 

 bilities of a subdivision of the Equisetales, we may regard the 

 genera Calamites, Archaeocalamites, Equisetites, Equisetum^ 

 Phyllotheca and Schizoneura as so many members of the Equi- 

 setales, without insisting on a classification which cannot be 

 supported by satisfactory evidence. 



Our knowledge of Calamites is fairly complete. Abundant 

 and well-preserved material from the Coal-Measures of England, 

 and from Pernio- Carboniferous rocks of France, Germany and 

 elsewhere, has enabled palaeobotanists to investigate the ana- 

 tomical characters of both the vegetative and reproductive 

 structures of this genus. We are in a position to give a 

 detailed diagnosis of Calamitean stems, roots and strobili, and 

 to determine the place of this type of plant in a system of 

 classification. Calamites not only illustrates the possibilities of 

 palaeobotanical research, but it demonstrates the importance of 

 fossil forms as foundations on which to construct the most 

 rational classification of existing plants. The close alliance 

 between Calamites and the recent Equisetaceae has been 

 clearly established, and certain characteristics of the former 

 genus render necessary an extension and modification of the 

 definition of the class to which both Calamites and Equisetites 

 belong. The Calamites broaden our conception of the Equi- 

 setaceous alliance, and by their resemblance to other extinct 

 Palaeozoic types they furnish us with important links towards 

 a phylogenetic series, which the other members of the Equise- 

 tales do not supply. 



From the Upper Devonian to the Permian epoch Calamites 

 and other closely related types played a prominent part in the 

 vegetation of the world. We have no good evidence for the 

 existence of Calamites in Triassic times ; in its place there were 



