OF THE FOSSIL PLAHTS OF THE COAL-MEASURES. 
101 
distinct genus, has strongly marked features of aliiniLy in the structure of its stem 
to the Sphenophylloid type. Hence all these plants may be recognised as constituting 
one of the several groups which collectively form the great family of the Calamarke, 
and of which family the modern genus Equisettim, instead of being the central type, 
is but a poor, feeble, and degraded member. 
Calamites. 
My attention has long been drawn to tlie fact that, whilst sandstone casts of 
considerable dimensions, moulded in the interiors of the tistular medulhe of 
Calamitean stems are notoriously common, similar casts of the interiors of the smaller 
branches are extremely rare. Fig. 20, which represents (natural size) the interior 
cast of a single internode of such a branch is the smallest one preserving the 
common features of cylindrical form, longitudinal ridges and furrows, and, at the 
two ends, the two nodal transverse constrictions, that I have met with during 
fifty years of research. I have thought it desiral)le to demonstrate the cause of 
this state of things. Tlie example here figured is '5 of an inch in diameter. 
Comparing a medulla of this size with those of other decorticated specimens in my 
cabinets, in which the exogenous woody zone is preserved, I have no douljt that fig. 20 
belonged to a branch or stem that was at least an inch in diameter, independently of 
its thick cortical zone. Seeing that I possess sections of decorticated Calamitean 
twigs not more than '03 in diameter, myriads of similar ones must exist, but which 
are never represented by these sandstone casts. I possess a very fine leaf-bearing 
branch in sandstone, for which I am indebted to my old friend Sir William 
Dawson, of which the diameter, when uncompressed, has been about ‘16 of an 
inch, including its cortex. It displays longitudinal internodal ridges, but these 
are but the irregular products of the shrivelling of a semi-herbaceous tissue, and 
have no affinity with the regular ridges and furrows of fig. 20. 
I have brought together in my cabinet a graduated series of specimens which 
illustrate and explain the conditions referred to. Two of these specimens are 
represented in figs. 21 and 22. Fig. 21 is the centre of a decorticated branch, 
the entire diameter of wliich is '25 of an inch, whilst that of its fistular medullary 
cavity, a, is about ‘05. This cavity is filled with inorganic matter, whilst at a a 
zone of medullary parenchyma is still preserved, which is about '005 in. in thickness. 
This medulla intervenes between the circumferential border of the inorganic cast, 
a, and the inner angles of the vascular wedges, h, b. This zone of undisturbed 
medullary tissue protects the periphery of the once plastic cast, a, from the 
pressure of the elongated inner angles of the woody wedges, h. It is to he 
remembered that in the very young Calamitean twigs this medulla is entire —not 
fistular ; but that at a very early stage of grow’th the centre of this entire medulla 
becomes ruptured. The inci})ient cavity thus formed increases in diameter as the 
branch grows, partly in consequence of the absorption of the medullary cells, and 
