Mr. Denny, the intelligent and indefatigable Curator of the Leeds Philosophical Society, has given the following admirable summary of the distinctive characters of the stems of most frequent occurrence in the Coal,[113] which will be found a useful guide to the collector. [113] On the Fossil Flora of the Carboniferous Epoch, with especial reference to the Yorkshire Coal-field. By Mr. Henry Denny, A.L.S. Proceedings of the Polytechnic Society of Yorkshire; for 1850. 1. _Sigillaria._--Stem furrowed, not branched, leaf-scars small, round, much narrower than the ridges of the stem. 2. _Favularia._--Stem furrowed, not branched, leaf-scars small, square, and as broad as the ridges of the stem. 3. _Lepidodendron._--Stem not furrowed, branched, covered with lozenge-shaped scars in quincuncial order, each having a papilla in the upper part; the upper portion of the stem and branches with simple linear leaves; the lower portion destitute of leaves. 4. _Halonia._--Stem not furrowed, branched, covered with indistinct rhomboidal marks, and tubercular projections disposed in quincunx. 5. _Knorria._--Stem not furrowed, branched, marked with projecting scars of petioles disposed spirally. 6. _Megaphyton._--Stem dotted, neither furrowed nor branched, leaf-scars very large, of a horse-shoe figure.[114] [114] Pictorial Atlas, pl. xxv. 7. _Bothrodendron._--Stem pitted, neither furrowed nor branched, scars of cones (?) obliquely oval. 8. _Ulodendron._--Stem neither furrowed nor branched, covered with rhomboidal marks; scars of cones (?) circular. The characters of the roots called Stigmariæ (_ante_, p. 134), and of the stems named _Calamities_ (ante, p. 107), and _Equisetites_ (_ante_, p. 106), are sufficiently distinct from the above to be easily recognized, I will briefly notice those not previously described. [Illustration: Lign. 41. Stems from the Coal Formation. Fig. 1.--Halonia regularis. _Coalbrook Dale._ 2.--Knorria Taxina. _Roof of the High-main Coal seam, Jarrow Colliery._ (_Brit. Foss. Flor._) ] _Halonia._--The specimens usually seen are mere sandstone casts having a thin carbonaceous crust; the stem is branched and beset with large elevated knobs, or subcortical protuberances, as shown in _fig. 1, Lign. 41_. These plants appear to be closely related to the Lepidodendra; their mode of branching is shown in a beautiful specimen (in the museum of the Leeds Philos. Soc.) figured and described by Mr. Denny, which is also remarkable because it indicates the probability that the Haloniæ, and the fossil stems, termed Knorriæ, are identical; for the specimen in question, which in its branches is unquestionably of the former type, has the base of the stem impressed with the leaf-scars of the latter.