XVIl] STIGMARIA 229 



an argument in favour of the view that the beds containing 

 such specimens are old surface-soils. 



Stigmaria usually shows regular dichotomous branching, the 

 arms spreading horizontally or slightly downwards and always 

 arising from four main branches in the form of a cross (fig. 207). 

 The most remarkable specimens found in England are described 

 by Williamson 1 in his monograph of Stigmaria. One of two 

 large casts found near Bradford in Yorkshire, and now in the 

 Manchester Museum, shows four large primary arras radiating 

 from the base of an erect stump 4 feet in diameter. Each 

 arm divides a short distance from its base into two, and the 

 smaller branches extend almost horizontally for several feef. 



An illustration published by Martin in 1809' shows a 

 characteristic feature of Stigmarian casts, namely the presence 

 of a smaller axis, usually occupying an eccentric position 

 inside the larger. This represents the cast of the fairly broad 

 parenchymatous pith which, on decay, left a space subsequently 

 filled by sand or mud : at a later stage the surrounding wood 

 and cortex were removed and the cavity so formed was 

 similarly filled. A thin layer of coal formed by the carbonisa- 

 tion of some of the tissues frequently surrounds the medullary 

 cast, and Steinhauer, whose account of the genus is much 

 fuller and more scientific than those of other earlier and 

 many later writers, recognised the true nature of this internal 

 cast. Artis^ regarded it as the remains of a young plant, which 

 he described as " perforating its parent," at length bursting it 

 and assuming its place, a gratuitously drastic interpretation. 



In 1838' Lindley and Hutton figured a partially petrified 

 specimen of Stigmaria obtained by Prestwich from Carbo- 

 niferous rock of Shropshire. This example showed a fairly 

 broad cylinder of secondary wood penetrated by medullary 

 rays. The medullated stele consisted of a pith surrounded by 

 a small amount of primary xylem and by a cylinder of 



1 Williamson (87) A. 



2 A similar example, now in the Bergakademie of Berlin, has been described 

 by Potoni6 (90) A. ; see also a note on the German specimen by Seward (91). 



3 Martin (09) A. PI. xii. <> Artis (25) A. 

 5 Lindley and Hutton (38) A. PI. clxvi. 



