198 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
That the Sigillarice were Lepidodendroid is a conclusion that has been already arrived 
at, first by Dr. Hooker and afterwards by Mr. Binney and Mr. Carruthers ; but the 
facts upon which this conclusion was based by these writers appear to me insufficient to 
furnish a demonstration of this affinity, since no example of a true Sigillaria in which 
the internal structure is preserved appears to have been hitherto described. Mr. Binney 
has described some plants* which he believes to be true Sagillarim ; but I agree with 
Mr. Carruthers, who has pointed out that one of these f is a true Lepidodendron. 
Another $ is a very curious and distinct plant of which I have sections, but which I 
have, as yet, failed to interpret ; whilst the remaining plates refer to a plant which I 
shall notice further in this memoir, and which may be a Sigillaria ; but I fear that we 
have not as yet sufficient evidence to render justifiable the conclusion that it is so. 
Mr. Carruthers informs me that since his several memoirs referring to this subject 
were published, he has obtained such a Sigillaria , which he is about to describe ; but 
not having seen the specimen I am unable to form any opinion respecting it. 
My object in this memoir is to describe and illustrate the structure and affinities of 
several genera respecting which there is no longer any ground for doubt, and also to 
demonstrate the successive steps by which we ascend from the lowest type of Lepido- 
dendron to stems which, as Brongniart has truly concluded, are furnished with an 
exogenous woody cylinder, richly supplied with medullary rays. For this purpose I 
shall take as my point of departure the Lepidodendron figured by Mr. Binney, already 
referred to, but which, in his specimen, lacked the outermost epidermal layer. This 
plant has also been described by Mr. Carruthers §, who regards it as identical with the 
Lepidodendron selaginoides of Sternberg. I owe some apology to the latter gentleman 
for redescribing a plant which, so far as he went, he has described so accurately ; but 
to do so is indispensable to the object which I have in view, and which includes points 
not referred to in his memoir, as well as some others on which I am constrained to arrive 
at a different conclusion from his. He carefully abstains in his memoir from employing 
the terms medulla or pith, wood or ligneous cylinder, and bark ; whereas I am satisfied 
that these three portions, characteristic of an exogenous growth, are to be discovered in 
the entire series of these plants. This threefold division is least conspicuous in the type 
just referred to ; but a gradation of forms leads us from that type up to others in which 
the tripartite distinction is too remarkable to be overlooked. I have no doubt in my 
own mind respecting the existence of these divisions throughout the entire series ; con- 
sequently in this memoir I shall speak of the medullary, ligneous, cortical, and epidermal 
layers, and I shall also always employ the same letters to indicate what I believe to be 
homologous structures in the various plants described. 
* “ A Description of some Fossil Plants, showing structure, found in the Lower Coal-seams of Lancashire 
and Yorkshire,” Phil. Trans. 1865, p. 579. 
t Log. cit. pi. 35. figs. 5, 6. J Log. cit. pi. 34. figs. 1, 2, 3. 
§ “ On the Structure of the stems of the arborescent Lycopodiaceas of the Coal-measures,” by W. Carruthers, 
Esq., F.L.S., F.G.S., Botanical Department, British Museum (Monthly Microscopical Journal, London, October 
1869). 
