PTEROPSIDA 121 



similar to Metaclepsydropsis. Its stem anatomy differed, 

 however, in that the inner region of xylem consisted entirely 

 of tracheids. The tracheids of the outer region were arranged 

 in radial rows, but are nevertheless beheved by some to have 

 been primary in origin. The petiolar trace had a very narrow 

 'waist', with the result that it appeared X-shaped in cross 

 section. 



Dineuron ellipticum, also from Pettycur, on the other hand 

 had no *waist' at all in the petiolar trace, which was elhptical 

 in cross section. 



In all three of these Lower Carboniferous genera, the 

 origin of pinna traces was the same, suggesting that pairs of 

 lateral pinnae were arranged alternately along the petiole, 

 or phyllophore. The frond was thus a highly compound one 

 whose components formed a three-dimensional structure. 

 However, it had not been realized just how complex they 

 were until the important discovery, in 195 1, of a mummified 

 specimen of Rhacophyton zygopteroides.^^ That this plant 

 belonged to the Zygopteridaceae was established by examin- 

 ing its internal anatomy. It had a fairly stout stem bearing 

 roots and spirally arranged fronds. The lowermost fronds 

 were sterile and were bipinnate, the pinnules consisting of 

 dichotomous branchlets, apparently without any flattening 

 to form a lamina. The fertile fronds were much larger and 

 more complex, and had pairs of pinnae arranged alternately, 

 just as had been deduced for Metaclepsydropsis from a study 

 of petrified material. Each of the paired pinnae was similar 

 in its branching to a sterile frond. Whereas the lower pinna 

 pairs had branched aphlebiae, these were replaced in the 

 higher pinna pairs by profusely branched structures bearing 

 numerous terminal sporangia. These were about 2 mm long 

 and were without any specially thickened annulus. 



At least eight species are known of the Upper Carbonifer- 

 ous genus Ankyropteris, which derives its name from the 

 fact that the petiolar trace in some species, e.g. A. westfali- 

 ensis, was shaped hke a double anchor (Fig. 17O). In some 



