326 FILICALES [CH. 



are justified in tracing the Osmundaceae as far back as the 

 Lower Carboniferous period, some of the characteristics of the 

 family were already foreshadowed in rocks of this age. 



Through a fortunate accident of preservation, unequivocal 

 evidence of the existence of Osmundaceae in the Palaeozoic era 

 is supplied by the Russian Upper Permian genera Zalesskya 

 and Thamnopteris. 



Zalesskya. 



This generic title has been instituted by Kidston and 

 Gwyime-Vaughan^ for two Russian stems of Upper Permian 

 age, one of which was named by Eichwald^ Chelepteris gracilis, 

 but the probability that the type of the genus -Chelepteris is 

 generically distinct from Eichwald's species necessitated a new 

 designation for the Permian fern. 



In habit the stem of Zalesskya resembles that of an 

 Osmunda or a Todea, but it differs in the possession of a stele 

 composed of a continuous cylinder or solid column of xylem 

 surrounded by phloem, and by the differentiation of the xylem 

 into two concentric zones. The leaves are represented by 

 petiole-bases only ; the sporangia are unknown. The stem and 

 leaf-base anatomy fully justifies the inclusion of Zalesskya in 

 the Osmundaceae. 



Zalesskya gracilis (Eichwald). Fig. 248. 



The type-specimen is a partially decorticated stem, 'from 

 Upper Permian beds in Russia, provided with a single stele, 

 13mm. in diameter, surrounded by a broad thin- walled inner 

 cortex containing numerous leaf- traces and occasional roots : this 

 was doubtless succeeded by a sclerotic outer cortex. In its 

 main features Zalesskya gracilis agrees closely with Z. diploxylon 

 represented in fig. 249. The stele consists of a continuous 

 cylinder of xylem exhibiting a fairly distinct diflferentiation 

 into two zones, (i) a broader outer zone of narrower scalariform 

 tracheae {x ii, fig. 248) in which 20 to 25 protoxylem strands (px) 



1 Kidston and Gwynne-Vaugban (08). " Eichwald (60). 



