IV] FOSSIL CASTS. 73 



the formation of casts ^ The outer surface with the characteristic 

 spirally arranged circular depressions, represents the wrinkled 

 bark of the dried plant ; the smaller cylinder, on the left side 

 of the upper end (fig. 12, 2, p), marks the position of the pith 



Fig. 12. Stigmaria ficoides Brongn. 1. Side view, showing wrinkled surface 

 and the scars of appendages. 2. End view (upper) showing the displaced 

 central cylinder; p, pith, x, xylem, r, medullary rays. 3. End view (lower). 

 From a specimen in the Woodwardian Museum. \ nat. size. 



surrounded by the secondary wood, which has been displaced 

 from its axial position. The pith decayed first, and the space 

 was filled in with mud ; somewhat later the wood and cortex 

 were partially destroyed, and the rod of material which had 

 been introduced into the pith-cavity dropped towards one side 

 of the decaying shell of bark. 



As the parenchymatous medullary rays readily decayed, the 

 mud in the pith extended outwards between the segments of 

 wood which still remained intact, and so spokes of argillaceous 

 material were formed which filled the medullary ray cavities. 

 The cortical tissues were decomposed, and their place taken 

 by more argillaceous material. At one end of the specimen 

 (fig. 12, 3) we find the wood has decayed without its place being 

 afterwards filled up with foreign material. At the opposite 



^ The British Museum collection contains a specimen of Stiginaria preserved 

 in the same manner as the example shown in fig. 12. 



