xxxvn] 



WILLIAMSONIA 



429 



an involucrum. The base of the fossil is characterised by an 

 annular zone formed of crowded, radially disposed, narrow 

 ridges now known to be casts of interseminal scales. At the 

 outer edge of this annular area impressions of the peltate 

 ends of interseminal scales are not infrequently preserved. 

 Fig. 545 is a photograph of one of Nathorst's preparations show- 

 ing the very great similarity between a 

 micropylar tube of W. gigas and the 

 corresponding structures in Cycadeoidea. 

 The small micropylar tubes are surrounded 

 by 5 — 6 polygonal expanded apices of 

 interseminal scales as in Cycadeoidea (fig. 

 515; cf. also fig. 563), and the apex of 

 each peltate distal end projects shghtly 

 as a central papilla composed of more 

 strongly cuticularised cells. In most 

 specimens the megasporophylls and inter- 

 seminal scales (sterile megasporophylls) 

 are preserved only as an annular zone at 

 the base of the receptacle (fig. 548, as), 

 but it is clear from some specimens of 

 W. gigas and other species figured by Saporta^, Nathorst*, 

 and Krasser^ that originally the whole surface of the pjrriform 

 axis was beset with these organs which fell off, presumably, 

 when the seeds had reached maturity. No satisfactory examples 

 of seeds have been found in English specimens. Krasser has 

 described some specimens of Williamsonia from Jurassic rocks 

 in Sardinia to which he assigns some associated seeds, but, as 

 he admits, there is no proof of any connexion. In some cases 

 a funnel-like depression is seen at the upper end of a strobilus of 

 W. gigas (fig. 546, B, C, a) identical in the occurrence of radially 

 disposed ridges with the annular zone at the base and due to the 

 preservation of interseminal scales and aborted megasporophylls 

 in the upper part of the receptacle : in this region also the impres- 

 sions of polygonal apices of the scales are sometimes found. The 

 probability is that while the greater part of the armour of scales 

 and seeds was thrown off, at the upper and lower end of the 

 1 Saporta (91) PI. 248. « Nathorat (09) PI. m. ' Krasser (12). 



Fig. 545. Williamsonia 

 gigas. Micropylar tube. 

 (After Nathorst.) 



