CHAPTER XXVIII. 
II. SPHENOPHYLLALES. 
Tus second phylum of the sporangiophoric Pteridophytes includes the 
title-family of extinct fossil plants, the Sphenophylleae, and associated with 
them, though perhaps somewhat aloof, as differing in certain important 
features, is the living family of the Psilotaceae: this contains the genera 
Psilotum and Zmesipteris. Certain other imperfectly known fossils may 
also find their best place in this relationship. The Sphenophyllales are 
characterised by having a dominant axis, with protostelic structure, which 
bears leaves of moderate size, with more or less furcate branching, and 
arranged either in whorls (Sphenophyl/um) or alternate (Psilotaceae). An 
important distinctive character is the insertion of the sporangiophores not 
directly on the axis, but upon the appendages: they are thus marked 
off clearly from the Equisetales, notwithstanding that they have many 
points of resemblance to them: these points are more marked in the 
Sphenophylleae, while the relation of the Psilotaceae is rather towards 
the Lycopodiales. The whole phylum thus occupies an intermediate, or 
perhaps a central position, which gives its study a very special interest. 
A. SPHENOPHYLLEAR. 
This ancient and long extinct family is represented according to present 
knowledge by the undivided genus Sphenophyllum: but associated more 
or less distinctly with it is the complex strobilus known as Checrostrobus. 
The Sphenophylleae as at present known dated from the Calciferous 
Sandstone series of the Lower Carboniferous formation, and extended 
upwards to the Permian. They were plants of straggling habit, with the 
usual vegetative region preceding the spore-producing parts: these were 
commonly borne upon definite terminal strobili, but at least one case is 
known where the definition of the vegetative and reproductive regions was 
less clearly marked. In the case of Chetrostrobus the vegetative region 
is still unknown. 
