FOSSIL PLANTS. 51 



on the Lepidodendroid leaf-cushion, there is frequently a 

 triangular depression immediately above the upper limit 

 of the leaf-scar ; this has been regarded by some as the 

 position occupied by a ligule, but Williamson does not 

 accept this view, and prefers to speak of the structure 

 which appears at the surface of the cushion in the form 

 of triangular depression as the "adenoid organ". 1 It is 

 well shown in some of Williamson's figures of Lepidoden- 

 dron Harcourtii leaves ; and Potonie also figures and 

 describes the same structure, and accepts the suggestion of 

 its ligular nature. 2 



Another important question on which light is thrown 

 by Williamson's memoir is the connection between Halonia 

 and Ulodendron ; Brongniart's original distinction between 

 the two genera, based on the numerous rows of quincuncially 

 arranged tubercles in the former, and the two vertical rows 

 of the latter, is shown to be no longer tenable. In both 

 genera the tubercles are evidently of the same nature, and 

 represent the points of attachment of strobili. In some 

 forms of Lepidodendroid stems the strobili were sessile, 3 in 

 others they terminated longer or shorter leafy axes ; 4 in 

 the former case, the base of the strobilus would press 

 against the surface of the stem, and produce a pressure 

 disc such as we are familiar with in typical Ulodendron 

 specimens. If the strobili were not adpressed to the stem, 

 there would be no pressure disc formed, and the result 

 would be the Halonial form of branch. 



Williamson's excellent figures and descriptions should 

 be consulted for a fuller account of this interesting ques- 

 tion. 



Another communication from Potonie 5 deals with the 



1 Phil. Trans., loc. cit., p. 12. 



2 Since this was written, Prof. Williamson has told me that the 

 figures of Lepidodendroid leaf-cushions, in his recent memoir, represent 

 the structures in an inverted position ; his interpretation of the facts 

 agrees, therefore, with that of Potonie and others. 



3 Phil. Trans., loc. cit., pi. vi., fig. 26 a. 



4 Ibid., pi. v., fig. 27 a. 



5 Per. deutsch. hot. Ges., Jahrg. ii., Heft 8, 1893, p. 484. 



