502 Prof. Williamson on Lepidodendra and Sigillariae. [June 15, 



Of this family numerous other modifications are described. Thus Ulo- 

 dendron and Halonia, very closely allied, if not identical genera, have a 

 structure closely corresponding with that of Lepidodendron Harcourtii, 

 since they possess a very distinct cellular medullary axis enclosed within 

 the ring of medullary vessels, and, besides, exhibit the enclosing ligneous 

 zone at its minimum stage of development. The remarkable scars of Vlo- 

 dendron and the tubercles of Halonia appear to have had their most pro- 

 minent surfaces composed of the true bark-layer deprived of its epidermal 

 bast and parenchymatous layers, which surround these structures but do 

 not wholly enclose them. These characteristic structures are believed to 

 have supported special organs, into which the epidermal layer of the stem 

 has been prolonged, and which the author believes to have been reproduc- 

 tive cones. Favularia corresponds very closely, so far as its cortical layer 

 is concerned, with those already described ; and as Brongniart's Sigillaria 

 elegans is an unquestionable Favularia, the entire series of this subgenus 

 is brought into the closest relationship with the plants described. But 

 the author has further met with some important examples, showing that 

 the stem supported verticils of organs that were neither leaves nor branches, 

 but which are believed to have been cones, thus bringing to light an addi- 

 tional indication of affinity between Favularia, Halonia, and TJlodendron. 



Well-marked examples have also been obtained from the Lancashire 

 Lower Coal-measures, the source whence all the specimens described have 

 been obtained, of the outer cortical layers of true Sigillariae. These spe- 

 cimens demonstrate that the bark of these plants is of the true Lepido- 

 dendron type. No example of an unquestionable Sigillaria in which the 

 central woody axis is preserved has yet been seen by the author. 



Stigmaria is shown to have been much misunderstood, so far as the 

 details of its structure are concerned, especially of late years. In his 

 memoir of Sigillaria elegans, published in 1839, M. Brongniart gave a 

 description of it, which, though limited to a small portion of its structure, 

 was, as far as it went, a remarkably correct one. The plant now well known 

 to be a root of Sigillaria, possessed a cellular pith without any trace of a 

 distinct outer zone of medullary vessels, such as is universal amongst the 

 Lepidodendra. The pith is immediately surrounded by a thick and well- 

 developed ligneous cylinder, which contains two distinct sets of primary 

 and secondary medullary rays. The primary ones are of large size, and 

 are arranged in regular quincunctial order ; they are composed of thick 

 masses of mural cellular tissue. A tangential section of each ray exhibits 

 a lenticular outline, the long axis of which corresponds with that of the 

 stem. These rays pass directly outwards from pith to bark, and separate 

 the larger woody wedges which constitute so distinct a feature in all trans- 

 verse sections of this zone, and each of which consists of aggregated 

 laminae of barred vessels disposed in very regular radiating series. The 

 smaller rays consist of vertical piles of cells, arranged in single rows, and 

 often consisting of but one, two, or three cells in each vertical series ; 



