xvi FOSSIL ARCHEGONIATES 587 



showed a secondary growth in thickness which is almost en- 

 tirely wanting in all the living species. These great horse- 

 tails rapidly disappear from the later formations.. 



The genus Equisetites has also been reported from the later 

 Palaeozoic formations, but there seems some question whether 

 these are not more nearly allied to the Calamariaceae. 



Two other Mesozoic genera have been described, which 

 probably are allied to the Equisetaceae, but they are too imper- 

 fectly known to make this at all certain. These are Phyllo- 

 theca and Schizoneura. Both v had the characteristic jointed 

 stems with the leaves more or less completely united into sheaths 

 about the nodes, as in Equisctum, but the leaves were better 

 developed than in that genus. (See Seward (i), Figs. 

 68, 69). 



The oldest known member of the class, Aster ocalamites, 

 has been found in the middle Devonian. In the later Devonian 

 the true Calamites appear and increase rapidly in numbers dur- 

 the Carboniferous, disappearing before the Trias, when their 

 place is taken by forms closely allied to the living Equisetaceae. 



SPHENOPHYLLALES 



The Sphenophyllales comprise a small number of extremely 

 peculiar fossils, belonging mainly to the Palaeozoic, but extend- 

 ing into the earlier Mesozoic also. Aside from the fructifica- 

 tions which have been attributed to them, and some of which 

 have been described under other generic names, they have all 

 been referred to a single genus, Sphenophyllum. They were 

 plants with slender, jointed stems, resembling more nearly 

 those of the Equisetaceae than any other living Pteridophyte. 

 About the nodes were whorls of wedge-shaped leaves, in some 

 cases dichotomously divided, and not unlike those of Archcuo- 

 calamites. (Potonie (3), Figs. 172-75). 



The anatomy of the stem is very different from that of the 

 true Equisetales, having a single central vascular cylinder, in 

 some respects like that of the typical Lycopods. It has been 

 compared to that of Psilotum or Tmesjpteris. (Scott (i), 

 Figs. 34, 35). 



The fructifications of undoubted species of Sphenophyllum' 

 have been found, and the fossils described under the names 

 Boiwncmitcs and Cheirostrobus are supposed to have been the 



