366 CALAMITES. [CH. 



and multiplying the breadth by 27 ; the explanation being that 

 a zone of wood 27 mm. in thickness is reduced in the process of 

 carbonisation to a layer 1 mm. thick. 



The breadth of the coal on the same form of cast may vary 

 considerably; on this account, and for various other reasons, 

 such a character can have but little value. Our knowledge of 

 anatomy may often help us to interpret certain features of 

 internal casts and to appreciate apparently unimportant details. 

 One occasionally notices that a Calamite pith-cast has large 

 infranodal canals, and in some specimens each internodal ridge 

 may be traversed by a narrow median line or small groove; 

 large infranodal canal casts suggest the type of stem referred 

 to the subgenus Arthrodendron, and the median line on the 

 ridges may be due to bands of hard tissue in each principal 

 medullary ray. 



In attempting to identify pith-casts the student must keep 

 in view the probable differences presented by the branching 

 rhizome, the main aerial branches and the finer shoots of- the 

 same individual. The long internodal ridges of some casts may 

 be mistaken for the parallel veins of such a leaf as Cordaites, a 

 Palaeozoic Gymnosperm, if there are no nodes visible on the 

 specimen. The fossil figured by Lindley and Hutton^ as 

 Poacites, and regarded by them as a Monocotyledon, is no 

 doubt a portion of a Calamite with very long internodes. An 

 interesting example of incorrect determination has recently 

 been pointed out by Nathorst^ in the case of certain casts 

 from Bear Island, originally described by Heer as examples of 

 Calamites; the vertical rows of leaf-trace casts on a Knorria 

 were mistaken for the ribs of a Calamite stem. The specimens 

 in the Stockholm Museum fully bear out Nathorst's inter- 

 pretation. The undulating course of internodal ridges and 

 grooves is not in itself a character of specific value. If a 

 Calamite stem were bent slightly, the wood and medullary-ray 

 tissues on the concave side might adapt themselves to the 

 shortening of the stem by becoming more or less folded, and a 



1 Lindley and Hutton (31), PI. cxliib. The original specimen is in the 

 University College Collection, London. 



2 Nathorst (94), p. 56, PI. xv. figs. 1 and 2. 



