■XXXII] PTYCHOXYLON 187 



adopt the name Lyginodendron robustum, though I suggested 

 that possibly Cycadoxylon might be the more appropriate genus. 

 Subsequently Scott^^ proposed the substitution of Cycadoxylon 

 for Lyginodendron. 



The type-specimen consists of secondary xylem agreeing 

 closely in structure with Lyginopteris and recent Cycads: the 

 pith, 2-9 cm. in breadth, is incompletely preserved ; there is a 

 narrow band of centripetal xylem ^ at the periphery of the peri- 

 medullary region and close to the inner face of the main mass of 

 wood (fig. 453, C ; the black line marks the position of the centri- 

 petal xylem). Nests of sclerous tissue and secretory canals are 

 scattered in the medullary parenchyma and deeper in this region 

 are arcs of secondary parenchyma, possibly periderm. In places 

 the centripetal and centrifugal xylem are in contact and occasion- 

 ally the tapered ends of the rows of centrifugal tracheids merge 

 into groups of primary xylem elements. The preservation- in the 

 central region is far from complete, and although the occurrence 

 of primary xylem groups is probable it cannot be said to be 

 positively established. At the inner edge of the centrifugal 

 xylem and in tangential longitudinal sections a few leaf-traces 

 are seen, but nothing is known as to the nature of the leaf-traces 

 in their course beyond the stele nor have we any data with regard 

 to the leaves or reproductive organs. 



This older species differs from Cycadoxylon Fremyi in the 

 Umitation of the centripetal xylem to the outer portion of the 

 pith and in the presence of sclerous nests in the medullary region, 

 though the latter character is probably of no great taxonomic 

 value. Cycadoxylon robustum approaches more closely to Lygino- 

 pteris, and although the differences are sufiicient to justify a 

 distinctive generic name, there can be little doubt as to a fairly 

 intimate relationship between this type of Cycadoxylon and 

 Lyginopteris. 



Ptychoxylon. Eenault. 



Ptychoxylon^ Levyi Renault. Like many Palaeozoic genera 



founded on anatomical features, Ptychoxylon* is represented only 



1 Scott (00) B. p. 364. 



^ A second band is sometimes present. Scott (00) B. p. 484. 



» irri^, a fold. * Kenault (89); (96) A; (93) A. PI. lxix. 



