508 DR. NEWELL ARBER AND MR. F, W. LAWFIELD ON THE 
and Jongmans *, an advance copy of the text of which we have had an oppor- 
tunity of studying, thanks to the kindness of Dr. Kidston, the same 
arrangement still holds, though here for the first time a systematic attempt 
is made to discriminate between the two forms of preservation, 
It is hardly necessary, however, to point out that inerustations of the 
external features of the stems of these plants are of an entirely different 
morphological nature from the medullary casts. It was for this reason that 
the senior author t of the present paper proposed, in 1916, a new form genus 
Calamophloios for their reception, with the exception of the very distinet 
type Victyocalamites $. There are also other considerations to which we shall 
refer more fully at a later stage (p. 513) which lead us to adopt these genera 
here, though as will be seen they admittedly involve some difficulties as 
regards the specifie nomenclature. 
In the present paper we are concerned rather with the morphology than 
the taxonomy of these fossils, though since our conclusions on the latter 
.point differ somewhat from those of Kidston and Jongmans in their recent 
work (1915), we have added a short systematic revision of the British 
members of these genera which are best known to us. The synonymy of 
these types is pure in the sense that it excludes all examples which are 
simply pith casts or at any rate not incrustations of the true external 
surface. 
We take this opportunity of expressing our thanks to Dr. Kidston for the 
negative of one of the figures here reproduced. 
2. The External Morphology of Calamite Stems. 
It may perhaps be well to commence our study of the external morphology 
of Calamite stems by some brief reference to the pith casts, the characteristics 
of which are now, in the light of petrified material, very thoroughly under- 
stood. Pith casts always show the following features :— 
1. Nodes or constrictions of the stem at regular or irregular intervals, 
2. Internodes or intervals between nodes. These may be long as compared 
with their breadth or vice versa, regular or irregular in size, or again 
periodic in the arrangement of a series of longer or shorter internodes. 
3. Internodes ridged and grooved. The ridges may be of all degrees of 
fineness or coarseness, sharp or flat, broad or narrow. In most 
Calamites the ridges of one internode alternate in position with those 
of the internodes next above and below. 
In addition, true pith casts, as opposed to sub-medullary casts, may 
show : 
4. Infranodal canals. Small prints, formed by the openings of infranodal 
‘anals into the pith, one on each rib just below the nodal line. 
* Kidston & Jongmans (1915). T Arber (1916). t Arber (1912). 
