Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/348

 is the result of his independent observations. Binney obviously confuses the two kinds of horizontal structures when he speaks throughout his papers of medullary rays composed of " finely barred vessels." He neither figures nor describes any true medullary ray ; and it is certain from his drawings that they do not exist. This I have moreover established by the examination of numerous beautifully preserved specimens.

If, then, I am right in believing that the structureless spaces in the vascular cylinder of Sigillaria elegans do not represent medullary rays, the structure of the stem exactly agrees with that of the root. This is, moreover, precisely the structure of Lepidodendron, Halonia, Ulodendron, &c., so that I venture to refer, without any doubt, all these genera to the same great group of extinct arborescent Lycopodiaceoe.

Dr. Dawson describes the vascular cylinder of Sigillaria as much more complex than anything I have seen. He says it was double, the inner portion being composed of scalariform, and the outer of discigerous vessels. He has figured a specimen of a stem in which the two kinds of tissue exist together. The important discigerous tissue could not be altogether wanting in Brongniart's specimen if it were really a constituent part of the Sigillarian stem, even though, as Dr. Dawson suggests, it were a young stem or branch*.

Lyell, the privilege of examining Mr. Dawson's specimen of the Sigillaria figured in plate 17, vol. xv. of the Society's ' Quarterly Journal,' and which has supplied him with the data for his interpretation of the internal structure of the genus. It is unnecessary for me to say that the specimen is accurately figured and described ; but it yet appears to me more than doubtful that it belongs to Sigillaria. The fact that it was found within the trunk of an erect Sigillaria is not sufficient ; for we know that hollow Sigillarioe were often filled with foreign materials. Dr. Dawson has himself obtained from trees at the Joggins, where he found this specimen, the remains of a Lizard. In one of the stems from the volcanic ash-beds at Arran I have determined the existence of some eight or ten distinct stems belonging to several genera. The stem of the Sigillaria was erect ; and all these contained stems were arranged parallel to the stem itself.
 * Since writing the above I have had, through the kindness of Sir Charles

If the argument I have employed above is of any value, the axis described by Dr. Dawson must be foreign to the Sigillaria in which it occurred, seeing it contains structures which are not found in any of the numerous specimens of Stigmaria that have been examined. The root and stem, being homological structures, are in living plants composed of similar tissues. It is impossible to conceive of another state of things existing. But in Dr. Dawson's axis there are true medullary rays and true coniferous disk-bearing tissue, neither of which occur in Stigmaria. I have no doubt that Dr. Dawson's fossil is a true coniferous stem, having a large Sternbergia-pith. Like some other carboniferous Coniferoe. The scalariform vessels occupy the position of the "medullary sheath." Though true scalariform vessels have not been detected in the Coniferoe, they occur in the closely allied Gymnospermatous order Cycadeoe. The single series of disks on the walls of the vessels indicate an affinity between this fossil and the Abietineous Conifers ; there are, however, other characters which make it impossible at present to refer this and similar fossils with certainty to any recognized group. I believe Dr. Dawson's fossil to be the same as a stem which has for some time been engaging the attention of Prof. Williamson, for which he has proposed the name Calamopity, and a full description of which may be early expected from him.