of Goal-Measure Plants. 171 



both of convenience and of importance, as giving expression to the 

 fact that the Palaeozoic types have furnished us with evidence of 

 the best kind as to the common origin of the Ferns and Cycads. 

 While Lyginodendron exhibits certain definite fern-like characters, 

 the genus Heterangium exhibits a still more decided leaning to- 

 wards the Filicineae, the structure of the primary stele being an 

 important additional feature of marked Filicinean character. In 

 Megaloxylon we have a type of stem in which the primary xylem is 

 distinctly of the fern type ; the protoxylem is external, and not 

 internal as in Heterangium, but in recent ferns the xylem may be 

 endarch, mesarch or exarch, and no great importance from the point 

 of view of affinity to the ferns as a group should be attached to 

 this point. On the other hand, the mesarch structure of the 

 xylem of Heterangium, Lyginodendron and other Cycadofilices 

 affords an important Cycadean character which is not met with 

 in Megaloxylon. On the whole we may regard Megaloxylon Scotti 

 as a member of the Cycadofilices possessing certain features not 

 shared by other known genera, and as presenting in the structure 

 of the primary xylem a more definitely Filicinean than Cycadean 

 feature. 



Megaloxylon adds another connecting link between the Palaeo- 

 zoic Cycadofilices and recent ferns ; in anatomical characters the 

 two genera Lyginodendron and Heterangium approach most nearly 

 to the Osmundaceae and Gleicheniaceae respectively, in Megalo- 

 xylon, on the other hand, the structure of the primary xylem affords 

 evidence that the Lygodium type of stem was also represented in 

 the Cy cad-fern alliance which played so prominent a part in 

 Palaeozoic vegetation. 



III. Diagnosis. 



Megaloxylon, gen. nov. 



The stem (probably) monostelic ; the primary wood, which is 

 partly common and in part cauline, consists of tracheids and paren- 

 chyma ; the tracheids are for the most part very short and large, 

 with their walls covered with multiseriate polygonal bordered pits. 

 The secondary wood is of the Cycadean type, characterised by 

 broad medullary rays. The leaf- traces, which are exarch and 

 probably concentric in structure, consist of long tracheids asso- 

 ciated with xylem parenchyma ; each leaf-trace traverses the peri- 

 pheral region of the stele through several internodes, and finally 

 passes through the secondary wood in an obliquely horizontal 

 direction ; the primary vascular axis of the leaf-trace becomes 



