1880.] Organisation of Fossil Plants of the Coal-measures. 551 



types, viz., Sigillaria vascularis ; 2. Diploxyloid stems ; 3. Favularia 

 and Leiodermaria. At present he contends that only the second of these 

 forms has been discovered in Lepidodendron Harcourtii. He further 

 believes that there are three types of Lepidodendron known, repre- 

 sented by — 1. L. Rhodumense, with a solid central vascular axis, in 

 which the vessels are not intermingled with medullary cells ; 2. by L. 

 Harcourtii, in which the vascular axis is a cylinder surrounding a 

 cellular medulla ; and, 3. An undescribed plant, which he names 

 L. J utieri, in which the vascular cylinder is broken up into detached 

 bundles of vessels. 



The author of the present paper considers that the above conclusions 

 are not in accordance with the facts, and he proceeds to give his 

 reasons for this conclusion by demonstrating that we certainly have 

 two of the three supposed Sigillarian types represented in a young or 

 Lepidodendroid state — the first by Lepidodendron vasculare of Binney, 

 and the second by L. Harcourtii, whilst, judging from M. Renault's 

 own description, the L. Jutieri represents the third type. On the 

 other hand, the author believes that of M. Renault's three Lepidoden- 

 droid types the first is only a young state of the second, as illustrated 

 by the development of the Burntisland and Arran Lepidodendra 

 described in previous memoirs, whilst the able Frenchman appears not 

 to have been acquainted with the existence of the very characteristic 

 type of the L. vasculare of Binney. 



The author gives the series of facts upon which his opinions are 

 based by tracing the history of the development, first, of Lepidodendron 

 Selaginoides, the L. vasculare of Binney, and, second, of L. Harcourtii. 



Commencing with the declaration that the Lepidodendron vasculare 

 of Mr. Binney is but the young state of the Sigillaria vascularis of 

 the same author, he proceeds to show the successive stages by which 

 the vasculo- cellular medullary axis of the former becomes not only 

 enclosed within the exogenous cylinder of the latter, but that this 

 cylinder ultimately develops into a very conspicuous example of the 

 Diploxyloid form of stem. The growth of the exogenous cylinder 

 begins at one point of the periphery of the vasculo-medullary axis, 

 from which point it extends both laterally and radially. The exogenous 

 growth thus first appears in the transverse section of the Lepidoden- 

 droid twig as a small crescent, thickest at its centre, but whose two 

 horns creep gradually round the medullary axis, its constituent vas- 

 cular wedges also growing radially as the lateral growth advances, 

 until at length the exogenous zone forms a complete ring, enclosing 

 the vasculo-medullary axis, in which state it becomes the Sigillaria 

 vascularis of Mr. Binney and M. Renault. The various stages of this 

 growth are represented in the plates, in addition to which a section is 

 described and figured of a branch about to dichotomise, in which pro- 

 cess the vasculo-medullary axis has divided into two equal halves, one 



