220 MR It. KIDSTON AND MR D. T. GWYNNE-VAUGHAN ON 



Zalesskya gracilis, Eichwald, sp. (PI. I. figs. 1-3 ; PL II. figs. 4, 5, 8 ; 



PL III. figs. 9-20.) 



1860. Chelepteris gracilis, Eichwald, Leth&a Rossica, vol. i., p. 98., pi. iii. figs. 4-5 (1 non fig. 6). 

 1869. Chelepteris gracilis, Schimper, Traite d. paleont. veget., vol. i., p. 702, pi. H. fig. 4 (1 non 

 fig. 5). 



The description of this species is based upon a portion cut from the lower end of 

 the type specimen figured by Eichwald in his Lethsea Rossica, Taf. iii., fig. 4. A 

 photograph of the complete fossil by Mons. R. Koch was kindly forwarded to us by 

 Mods. Zalkssky, and is reproduced in Plate I., fig. 1. 



As seen in transverse section, the greater part of the fossil is in an excellent state 

 of preservation, although the central tissues have unfortunately disappeared. The 

 most conspicuous part of the stem is the very wide stele (fig. 2, st.), which is 13 mm. 

 in diameter. The stele is surrounded by a very wide thin-walled cortex, more of 

 which is preserved on one side than on the other. At its widest the cortex is about 

 20 mm. thick, but even there it certainly does not represent the whole thickness of 

 the stem. So far as it is present, the cortex is all thin -walled parenchyma except a 

 narrow border at the very periphery of its widest part (fig. 2, sc. C): At this region 

 the cortical elements somewhat suddenly become thick-walled and brown-coloured. In 

 all probability this represents the inner limit of an outer sclerotic cortical zone such as 

 occurs in the other Osmundacese, both fossil and living. The inordinate width of the 

 thin-walled inner cortex (fig. 2, i.e.) is a striking feature in our fossil. 



The cortex is traversed on all sides by a very large number of departing leaf-traces, 

 which arise from the stele in a close spiral. Here and there roots are also cut across 

 in various directions as they pass outwards. Each leaf-trace is accompanied by a 

 sheath of thin-walled inner cortex as it passes through the sclerotic zone (fig. 2, A). 

 Unfortunately, our sections do not include the extreme periphery of the stem, and there 

 is no indication of the leaf-bases becoming free from one another. 



The Structure of the Xylem. 



The xylem of the stele forms a broad and perfectly continuous ring surrounding a 

 central empty space (fig. 2). At one point in the stele a portion of it has been torn 

 away, but elsewhere the ring measures l - 8 to 1"3 mm. in thickness. It was un- 

 doubtedly still wider in the living condition, for its inner margin is bordered at many 

 places by the remains of crushed and flattened elements (figs. 3 and 4, x). Some six 

 or seven series of these crushed elements are present, and if they were imagined to have 

 retained their original form and size, the xylem ring would be about *9 mm. wider. 

 Therefore the maximum width of xylem for which we have actual evidence is about 

 3 "2 mm. But at the same time the complete ring is 12 mm. in diameter, and therefore 

 there remains a space 5*6 mm. wide in the middle of the stele still unaccounted for. 



