168 Mr Seward, Notes on the Binney Collection 



II. Comparison with other Genera. 



The secondary wood of the new genus, as already pointed out, 

 is of a type frequently met with in Palaeozoic genera ; it agrees in 

 the abundance of the medullary tissues and in the character of the 

 tracheids, with the wood of recent Cycads and with that of various 

 extinct genera which may be included in the class Cycadofilices of 

 Potonie 1 . In the Binney stem the wood is slightly more compact 

 than in the stems of Cycads or in many of the Palaeozoic types, 

 but the general structural features are essentially those which we 

 regard as characteristic of cycadean rather than coniferous plants. 

 In its detailed structure and in the broad continuous development 

 of the secondary xylem we may compare the new genus more 

 particularly with Lyginodendron robustum Sew. 2 and with the 

 stem named by Renault — on evidence which is hardly satisfactory 

 — Medullosa gigas 3 . 



The structure of the primary wood at once recalls that of 

 Heterangium and Medullosa anglica Scott 4 , which is practically 

 a polystelic Heterangium ; the parenchymatous pith of such genera 

 as Lyginodendron and Oalamopitys 5 affords an obvious distinction 

 between those types and the present genus. In the Heterangium 

 stele, as Williamson and Scott and Renault have pointed out, we 

 have tracheids which are confined to the stem and others which are 

 common to leaf and stem ; as the former authors have shown, there 

 is in Heterangium no sharp line between the metaxylem and the 

 primary xylem strands which constitute the leaf-trace bundles. 

 This is another feature shared by this genus and the one under 

 consideration. There are, however, certain important differences 

 which render it advisable to place the Binney stem in a genus 

 of its own. I propose to designate this new generic type of 

 Cycadofilices Megaloxylon, and the type-species Megaloxylon Scotti, 

 connecting it with the name of my friend Dr D. H. Scott, whose 

 researches have so materially extended our knowledge of the 

 Cycadofilices, and demonstrated the importance of this extinct 

 group from a phylogenetic standpoint. The primary perimedullary 

 strands in the stele of Heterangium, as also in Medullosa anglica, 

 are distinctly mesarch in structure ; the narrow spirally thickened 

 pro to xylem elements and the accompanying parenchymatous 

 elements occupy an internal position, and are not external or 

 exarch as in Megaloxylon. Another distinctive feature of the new 



1 Potonie' (97), p. 160. 



2 Seward (97). 



3 Renault (93), PI. lxxi. and (96), p. 297. 



4 Scott (99). 



5 Solms-Laubach (96), PI. iv. 



