ii6 SCIENCE PROGRESS. 



fern-like; the absence of a definite endodermis recalls the leaf 

 stalks of Marattiacece. The foliage is equally suggestive of 

 ferns in form and structure. In the roots, as regards the 

 primary structure, we have a further illustration of a fern 

 alliance, and especially with Marattiaceous genera. The 

 above comparisons are sufficient to conclusively demon- 

 strate the synthetic character of the fossil genus. 



The second form described by Williamson and Scott 

 must be dismissed in a few words. In 1871 certain stem 

 fragments were discovered amongst the well-known material 

 from Burntisland, which were subsequently referred to 

 Corda's genus Heterangium. Of this genus we now pos- 

 sess a comparatively full knowledge of two specific form, 

 H. Gricvii, Will, and H. tiliceoides, Will. The central 

 cylinder of the stem consists of an axial mass of primary 

 xylem traversed by anastomosing bands of parenchyma. 

 Towards the periphery of the central region distinct 

 groups of tracheae may be distinguished, containing proto- 

 xylem elements in the position characteristic of mesarch 

 bundles. As in Lyginodendron, these peripheral vascular 

 strands are true leaf-traces. There is a clearly marked 

 pericycle, and external to this a wide inner cortex of 

 parenchyma, containing characteristic horizontal plates of 

 sclerous tissue, followed by an outer cortex traversed to- 

 wards the periphery by radially disposed bands of scleren- 

 chymatous fibres. In most cases the primary xylem and 

 phloem tissues are surrounded by a zone of secondary wood 

 and bast with numerous medullary rays. The species 

 Heterangium tilicBoides is especially interesting as exhibit- 

 ing a remarkably perfect state of preservation of the sieve- 

 tubes with their lateral sieve-plates. The cambium is 

 occasionally extremely well preserved. The petioles and 

 pinnules are similar to those of Lyginodendron, but it has 

 not been possible to investigate their structure to quite the 

 same extent. 



Again, in its adventitious roots, Heferangizim agrees 

 closely with the preceding genus. With the exception of the 

 secondary xylem and phloem, the stem of Heterangium is 

 " essentially that of a monostelic fern of the Gleichenia 



