310 CALAMITES. [CH. 



principal medullary rays, mr, which become slightly narrower 

 towards the outside. The inner face of each of these wide rays 



Fig. 73. Transverse section of a Calamite stem, 

 mr, medullary ray. After Williamson. 

 X, X, xylem. (No. 1933 A.A. in the Williamson Collection.) 



has a concave form, due to the less resistent nature of the 

 medullary-ray cells as compared with the stronger xylem. 

 The regularly sinuous form of the inner face of the vascular 

 cylinder enables one to realise how the Calamite-casts (figs. 82, 

 99, and 101) have come to have the regular ridges and grooves on 

 their surface. The broad ridges on the cast mark the position 

 of the wide medullary rays, while the grooves correspond to 

 the more prominent ends of the vascular strands. The tissues 

 external to the wood have not been preserved in the example 

 shown in fig. 73. Some silicified specimens described by 

 Stur^ from Bohemia and now in the Museum of the Austrian 

 Geological Survey, Vienna, admirably illustrate the connection 

 between the surface features of a Calamite cast and the anatomy 

 of the stem. 



In the large section of a calcareous nodule diagram matically 

 shown in fig. 17 II. (p. 85) the secondary wood of a slightly 

 flattened Calamite is the most prominent plant fragment. 

 The pith-cavity has been almost obliterated by the lateral com- 

 pression of the woody cylinder, but the presence of the carinal 



1 Stur (87). 



