198 THALLOPHYTA. [CH. 



this form of structure was first supplied by Penhallow\ The 

 black network simulating parenchymatous tissue consists of 

 the substance of Nematophycus tubes which has been com- 

 pletely redistributed during fossilisation and collected along 

 fairly regular lines, as seen in figs. D and E. The original 

 structure has been almost completely destroyed, and the 

 material composing the walls of the large tubes has finally been 

 rearranged as a network, interrupted here and there by the 

 characteristic radial spaces which remain as evidence of the 

 original Nematophycus characters. It is possible in some cases 

 to trace every gradation from sections exhibiting the normal 

 structure through those having the appearance shown in 

 figs. D and E to others in which the structure is completely 

 lost. Penhallow describes this method of fossilisation in N. 

 crassus (Daws.) ; an examination of several specimens in the 

 National Collection leads me to entirely confirm his general 

 conclusions, and also to the opinion that N. Logani shows 

 exactly the same manner of mineralisation as N. crassus. The 

 chief point of interest as regards this method of preservation 

 lies in the fact that a fossil described by Dawson^ as Cellulo- 

 xylon primaevum, and referred to as a probable conifer, is 

 undoubtedly a badly preserved Nematophycus. Penhallow 

 examined Dawson's specimens and obtained convincing evi- 

 dence of their identity with certain forms of highly altered 

 Nematophycus stems. 



2. Nematophycus Stomei Barber. Fig. 40. The specimens 

 on which Barber^ founded this species w^ere obtained by 

 Mr Storrie from the Tymawr quarry near Cardiff, in beds of 

 Wenlock age. The fragmentary nature of the material is 

 largely compensated for by the excellence of the preservation. 

 We may briefly define the species as follows : 



The stem consists of separate interlacing undivided and 

 usually unbranched tubes of varying diameter. Spaces more 

 or less isodiametric in dimensions are scattered through the 

 tissue. The spaces constitute regions in which the tubular 

 elements branch freely. 



1 PenhaUow (89) and (93). 2 Dawson (81) p. 302. ^ Barber (92). 



