132 



FOSSIL PLANTS. 



tical with that of my spedraen (pi. xxviii, figs. 33 and 34). This, in its turn, only 

 differs from the more ordinary forms of Bijjloxylon in the crenulated outline which sepa- 

 rates the ligneous zone from the cylinder of medullary vessels, giving to the exterior of 

 the latter a fluted aspect, like that of a Calamite, but without the transverse nodal con- 

 striction of the latter genus. The Diploxylons, again, as I have already shown, shade 

 off into the ordinary forms oi Lepidodendron , and are, undoubtedly, Lepidodendroid plants 

 which have lost the central portion of their medullary axis. Remove the cellular tissues 

 from the centre of the plant which I have represented in figs. 8 and 9, and we have at 

 once the closest resemblance to Witham's Anahathra and Corda's Biploxylon, as well as 

 to those now under consideration. That Witham's plant is identical in type with mine 

 is further indicated by his tab. 8, fig. 12, where he exhibits one of the large compound 

 medullary rays shown in my pi. xxvii, fig. 23. The cellular tissues have not been 

 preserved in the medullary rays of Brongniart's Sigillaria eleyans ; but tab. 4, fig. 2, of 

 his memoir shows that his plant possessed similar ones to those which Witham and I have 

 figured. Further, the description which M. Brongniart has given of the outer bark and 

 epidermis of his plant, these being the only cortical elements remaining in his specimen, 

 would apply, with little or no alteration, to several of my Lepidodendroid and Sigillarian 

 types ; so that, whilst a really indisputable Siyillaria, like my pi. xxix, fig. 39, but in 

 which the woody axis is preserved m situ, is still an important desideratum, I have very 

 little doubt that, when discovered, it will be found to correspond with one of the several 

 varieties of Diploxylon. Most probably, also, my pi. xxv, fig. 8, representing one of 

 the extremes of the two types figured by Mr. Binney under the name of Sigillaria 

 vascularis, will also be found to belong to the same subtype of the same genus. Yet my 

 indefatigable friend informs me that his cabinet contains specimens in which the most 

 gradual transition can be traced, from the plant just referred to, to the Lepidodendron 

 selaginoides, the oppositely divergent form of the same group, hence his inclusion of 

 both under one common name. 



. . . . " Having thus obtained (Williamson, op. cit., p. 238) additional light 

 respecting the Diploxylons, I again turned to the more highly organised of the stems 

 described by Mr, Binney under the name of Slgillaria vascularis, and which I have 

 already represented in pi, xxv, figs. 8 — 11. I made a fresh series of carefully prepared 

 dissections, and succeeded in demonstrating the existence, in this plant, of a series of 

 primary and secondary medullary rays, the former containing large foliar bundles pre- 

 cisely identical with those of Diploxylon cycadoideum. I have not succeeded in dis- 

 covering in the former plant the cellular layer intervening between the medullary vascular 

 cylinder and the woody zone of the latter one. The large primary medullary rays are 

 composed of barred cells, which are sometimes mural, but more frequently prosenchy- 

 matous ; through the upper part of each of these large rays there proceeds a bundle of 

 true barred vessels. I have not succeeded in tracing one of these bundles to its medul- 

 lary extremity, consequently I cannot yet affirm how it originates; but I have seen 



