PSARONIUS 295 



were of great size, with a stem reaching, it is said, as 

 much as 60 feet in height. 



The arrangement of the leaves on these trunks 

 showed more variety than exists among living Tree- 

 ferns. Thus, in the genus Megaphytum of Artis, the 

 tall, somewhat slender stem bore the fronds in two 

 vertical series, as shown by the distichously arranged 

 leaf-scars, which were of large size, having a transverse 

 diameter of 8 or 10, and a height of 5 or 6 cm. There 

 was thus a certain superficial and deceptive resemblance 

 to large specimens of Ulodendron, with which the older 

 authors sometimes confused these stems. This phyllo- 

 taxis is apparently unknown among arborescent or erect 

 Fern-stems of the present age, though frequent in the 

 case of prostrate rhizomes. In other Palaeozoic Tree- 

 ferns the leaves were ranged in four vertical rows, while 

 in others, again, and these the most numerous, a spiral 

 polystichous disposition prevailed, as in the arborescent 

 Ferns of the present day. To guard against miscon- 

 ception, it may be well to say at once that the Palaeozoic 

 Tree-ferns only resembled those of the present day in 

 general habit ; their affinities doubtless lay in a direction 

 quite remote from that of the Cyatheaceae. 



The polystichous stems, in which the external surface, 

 bearing the foliar scars, is shown, are known by the 

 name of Caulopteris, while a separate generic name, 

 Ptychopteris, is applied to stems in another state of 

 preservation, in which the cortex has been stripped off 

 and the vascular system exposed. Lastly, yet another 

 genus had been founded for those stems of Tree-ferns 

 which are preserved in a petrified condition, so as to 

 retain their internal structure ; the latter, whatever the 



