250 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [March 24, 



it has arisen from some error of observation. I exclude from this 

 the specimen figured by Cotta under the name oi Stigmariajicoides, 

 the vascular tissue of which is composed of Cycadian-like or of dotted 

 vessels traversed by muriform cells of medullary rays ; for this is 

 certainly not a specimen of Stigmaria. 



Brongniart, in his memoir on Sigillaria elegans, figures a specimen 

 of Stigmaria from the cabinet of Robert Brown, which is now in the 

 collection of the British Museum, and which I have repeatedly and 

 carefully examined. In an enlarged representation of a portion of 

 the cylinder (pi. v. fig. 3) there is shown an axial vascular bundle 

 passing outwards through a mesh between the large wedges of the 

 cylinder. Several open spaces separating the parallel series of vessels 

 on the outer aspect of the cylinder are described as small medullary 

 rays ; but the examination of this and of an extensive series of other 

 specimens has convinced me that these are only disruptions of the 

 tissue produced along the line of least resistance from desiccation or 

 some other cause before or during the process of fossilization. 

 Brongniart was unable to detect any remains of structure in these 

 spaces. Such openings are altogether absent in many specimens which 

 I have examined ; and in one in the collection of the British Museum 

 they occur on the one half of the specimen and not on the other. 



Dr. Hooker, in his paper on the structure of Stigmaria, makes 

 no reference to the small cellular medullary rays of Brongniart, 

 which he evidently could not detect in his specimens, and which 

 are not indicated in the least in his beautiful drawings. He, how- 

 ever, describes structures to which he applies the name of medullary 

 rays. " The cylindrical axis," he says, " is divided into elongated 

 wedge-shaped masses, rounded at their posterior or inner extremity 

 by numerous medullary rays of various breadths, some much nar- 

 rower than the diameter of the tubes, others considerbly broader, 

 but none are conspicuous to the naked eye, except towards the outer 

 circumference. The medullary rays, even the narrowest, are tra- 

 versed by bundles of tubes half the diameter of the largest vessels 

 of the axis, or even less " (p. 434). In the explanation of plate ii, 

 fig. 6, he further says, " The vascular tissue is divided by broad and 

 narrow meduUary rays ;" and fig. 8 is a " highly magnified view of 

 one of the narrow medullary rays forming a bundle of tubes of less 

 diameter than those forming the wood." The structures described 

 in these extracts are evidently the vascular bundles which belong to 

 the axial appendages. These bundles take their origin in the inner 

 surface of the vascular cylinder, and pass upwards and outwards 

 through a mesh, which increases in size as it reaches the circum- 

 ference of the cylinder. The vascular bundle did not completely 

 fill the mesh, but was accompanied by a quantity of cellular tissue 

 which is very rarely preserved, the space occupied by it being gene- 

 rally filled with the amorphous material of the matrix in which the 

 specimen is preserved. This parenchyma is composed of roundish 

 cells. In the fern -stem a similar structure exists. The vascular 

 tissue of each frond has a parenchyma associated with it which is 

 continuous with the medulla. 



