74 FOSSIL PLANTS. 



III. General Observations on Lepidodendron, Stigmaria, and Halonia. 



The genus Lepidodendron, as shown by the Hesley Heath and Dudley specimens, 

 although they are very similar in their external characters with small specimens of SigU- 

 laria vascularis, differs much from them in its internal structure. The first-named plant 

 shows no trace of a radiating cylinder of barred tubes arranged in wedge-shaped masses, 

 or anything like medullary rays, whereas the last-named affords evidence of both these 

 important characters, and in those parts it is identical in structure with Stigmaria. 

 The specimens figured and described by me in the ' Quarterly Journal of the Geological 

 Society,' and in the ' Philosophical Transactions,' amply prove that Sigillaria was an exo- 

 genous tree, and had an internal vascular cylinder arranged in wedge-shaped masses, and 

 penetrated by medullary rays, as well as an external zone arranged in radiating series of 

 strong vascular-like tubes, or utricles, without any appearance of bars or discs, and pene- 

 trated by large vascular bundles. Both the large and small specimens were the same in 

 internal structure ; and the one could be shown to pass gradually into the other, the 

 rhomboidal scars of the former giving place to the irregular ribs and furrows of the latter. 



Professor Williamson has had in his possession for many years a specimen of 

 Stigmaria, figured and described by me in 1849, 1 which, although the medulla is absent, 

 distinctly shows in the inside of the cylinder, as left, the medullary rays traversing that 

 portion of the stem. Several specimens in my cabinet also distinctly prove the existence 

 of these vessels in Stigmaria. 



Sir William Logan, myself, and others have alluded to the occurrence of Stigmaria in 

 the floors of coal-seams. The specimens of this root there found are almost always in a 

 very indistinct and distorted condition, and the only characters generally to be seen are 

 the impressions or casts of some circular areolae and the traces of rootlets in the fire-clay. 

 Yet from these imperfect characters the whole of Stigmaria in this country have been 

 generally included in the species Jicoides. It has long been my opinion that the external 

 characters of this root are common to several other genera of aquatic plants besides 

 Sigillaria, and that such roots would have a great similarity in appearance from their 

 former soft and muddy habitats. Now, all the specimens of Stigmaria which I have seen, 

 and whose connection I have traced with upright stems of Sigillaria, have always been 

 of a comparatively large size, seldom if ever less than from two to three inches in diameter. 

 The Sigillaria at its base divides into four main roots, each of which bifurcates into two 

 secondary roots, which in their turn again dichotomize into two tertiary roots. Each of 

 these last present on their surface an irregularly ribbed and furrowed, or, more correctly 

 speaking, a gnarled appearance for a considerable distance outwards ; and this gradually 



' 'Quart. Journ. Geo!. Soc.,' vol. vi, p. 17. 



