438 Prof. W. C. Williamson, [Mar. 14, 



March 14, 1889. 

 Professor G. G. STOKES, D.C.L., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read : 



I. " On the Organisation of the Fossil Plants of the Coal- 

 measures ; Part XVI." By W. 0. WILLIAMSON, LL.D., 

 F.R.S., Professor of Botany in the Owens College, Man- 

 chester. Received March 5, 1889. 



(Abstract.) 



In this memoir the author first calls attention to detached observa- 

 tions made in his earlier memoirs relating to the manner in which a 

 medullary axis is developed in the interior of each of the primary 

 vascular bundles of the Carboniferous Lycopodiacae. He then traces 

 the changes undergone during the development of a small branch- 

 bundle in Lepidodendron Harcourtii. This is followed by a descrip- 

 tion of a small new species of Lepidodendron, which he named 

 L. mundum, and in which the peculiar development of the medulla is 

 clearly demonstrated. 



In a second new species, named Lepidodendron intermedium, a 

 peculiar and apparently early form of exogenous zone is shown to 

 exist. When describing, in his previous memoir, Part XI (see Plate 

 49, fig. 11), the stem now designated Lepidodendron fuliginosum, he 

 showed that, in it, we have an example of the most rudimentary and 

 least perfectly developed form of an exogenous zylem yet seen 

 amongst these Carboniferous Cryptogams. In this example, but a few 

 radiating laminae of vascular tissues make their appearance in the 

 innermost cortex. In the L. intermedium, now described, though these 

 few laminae are represented by a continuous cylindrical zone of 

 tracheids, and though the laminae are arranged in radial order, they 

 are still imbedded in a mass of cellular tissue, much in excess of what 

 constitutes the medullary rays in the higher types of Lepidodendroid 

 organisation. 



A fourth new species of Lepidodendron is described under the name 

 L. Spenceri, in young states of which no medulla is visible ; but in 

 its place a number of vertically elongated cells and imperfectly 



