SPENCER : ASTROMYELOX. 
441 
sometimes even in the same plant ; each wedge is composed of a 
number of laminae which are arranged in a radiating manner, and 
each lamina is again composed of a number of vessels which in- 
crease somewhat in size from the apex of the wedge outwards. 
In young specimens the vessels are larger and more various in 
size and form than in more mature forms. In older stems the 
vessels are smaller, and the exoginous zone is more firmly knit 
together by the growth of new cells among the older ones in a 
regular exoginous manner. The central medulla forms a solid 
pith, the cells generally largest in the centre, but become smaller 
as they approach the woody wedges and fill up the spaces between 
them, and being thin walled they give a light appearance to the 
pith, forming a strong contrast to the darker wedges of the ligne- 
ous zone, causing the ;t star-like" appearance from which the 
plant derives its name, "Astromyelon." 
Astromyelon differs also from Calamites in its mode of branch- 
ing. It has branches like those of ordinary forest trees, while in 
Calamites the branches were articulated to the stem, and had their 
origin solely in the thick bark. In the longitudinal sections of 
the two plants the pith is seen to be fistular in Calamites, with 
only a thin lining enveloping the inner ends of the woody wedges, 
whereas it forms a solid structure in Astromyelon ; the pith also 
extends to the branches which is not the case in Calamites. The 
structure of the pith is different in the two plants : in Calamites 
it partakes more of the character of cellular parenchyma, but in 
Astromyelon it is composed of oblong cells which form an inter- 
mediate stage between cellular tissue or parenchyma, and the 
higher stage of vessel structure, termed prosenchyma, the term 
used to indicate this higher stage of cell structure is parenchyma- 
tous. This mural parenchyma has been noticed by Professor Wil- 
liamson as forming a conspicuous feature in many of the fossil 
cryptogamic plants. 
In coal-balls two varieties of Astromyelon are found, namely, 
the normal one with the large star-like pith, and another in which 
