XXV] ANKYROPTERIS 455 



shown on the outer edge, a (fig. 313), of the antennae separated 

 fi:om the arcs of larger tracheae by a dark line which represents 

 a crushed band of delicate tissue. The spaces enclosed by the 

 incurved antennae are largely occupied by parenchymatous 

 ground-tissue. The cylinder of outer cortex consists internally of 

 comparatively thin-walled parenchyma succeeded externally by 

 a zone of dark and thicker- walled cells characterised by a fairly 

 regular arrangement in radial series, as if formed by a secondary 

 meristem; there is, however, no indication of a meristematic 

 layer. Below the small-celled epidermis are a few layers of 

 thinner-walled cells which are not arranged in radial series. 

 The structure of the outer part of the cortex is similar to that 

 in the petiole of recent species of Angiopteris (fig. 243, p. 319) 

 and Marattia, in which a more delicate hypoderm is succeeded 

 by a band of mechanical tissue. 



The rachis of this type of frond gives off two rows of lateral 

 branches fi-om the vascular axis, the plane of symmetry being 

 at right angles to the primary rachis. Each pinna bore at its 

 base two aphlebiae supplied with vascular strands from the 

 leaf-traces. 



We have no certain information in regard to the sporangia 

 of this species, but Scott points out that "pear-shaped sporangia, 

 with a very broad and extensive annulus, are commonly found 

 associated with Zygopteris bibractensis and Z. corrugata in the 

 petrifactions of the English Lower Coal-Measures'." 



Ankyropteris corrugata (Will). Figs. 312, B; 314—317. 



The stem of this type of Zygoptereae was described by 

 Williamson from the Lower Coal-Measures of Lancashire as 

 Rachiopteris corrugata and included by him in the sub-group 

 Anachoropteroides. The stele (fig. 314, B) is oval in transverse 

 section ; it consists of a cylinder of xylem tracheae enclosing a 

 central region occupied by parenchymatous tissue and scattered 

 narrow scalariform tracheae. The central tissue extends radially 

 in the form of narrow arms which reach almost to the outer 

 edge of the tracheal tissue and divide it up into 5 — 7 groups. 



' Scott (08) p. 322. 



