XVI] SIGILLARIA 201 



differences in the form of the leaf-cushion, but in some cases 

 the distinction becomes purely arbitrary. 



Immediately above the centre of the upper boundary of a 

 Sigillariau leaf-scar a ligule pit may often be detected, as 

 shown in fig. 195, I, and in some cases, e.g. a specimen figured 

 by Germar' (fig. 196, A) as Sigillaria spinulosa (identical with 

 S. Brardi), some circular scars with a central pit surrounded by 

 a raised rim occur on the surface of the stem, either singly 

 or in pairs, near the leaf-scars; these, it is suggested, may 

 represent the position of adventitious roots or, as Germar 

 thought, of some deciduous spinous processes. The leaf-scars 

 are frequently hexagonal in shape, with the lateral angles 

 either rounded (fig. 200, F) or sharply pointed (fig. 200, G, H) ; 

 each scar bears three smaller scars as in Lepidodendron, a 

 central circular, oval or crescentic leaf-trace scar and larger 

 oval or slightly curved scars formed by the two parichnos arms 

 (fig. 195, p). The larger size of the parichnos arms, the indi- 

 vidual cells of which may often be detected as a fine punctation, 

 is a distinguishing feature of the genus, but otherwise the 

 structure is very similar to that in Lepidodendron. As shown 

 in figs. 195, 200, F, G, the three scars may occur nearer the 

 upper than the lower margin of the leaf-base area. 



Lepidodendron Wortheni^ (fig. 196, E), described from North 

 America by Lesquereux^, by Zeiller^ from France, and by 

 Kidston' irom the Upper and Middle Coal-Measures of England, 

 may be quoted as a Lepidodendron bearing a close resemblance 

 to Sigillaria. The shoots bear cushions two or three times as 

 long as broad and without the usual median division, but with 

 numerous irregular and discontinuous transverse wrinklings. 

 Lepidodendron Peachii Kidston* affords another example of a 

 form agreeing both with Sigillaria and with Lepidodendron. 

 An Upper Devonian type described by White' as Archaeo- 

 sigillaria primaeva affords a striking instance of the combination 

 on one stem of Sigillarian and Lepidodendroid leaf-cushions. 



1 Germar (53). 



2 Cf. Lepidodendron Zeilleri, Zalessky (04) PL iv. fig. 1. 



3 Lesquereux (79) A. PI. lxiv. ^ Zeiller (88) A. PI. lxxi. 

 6 Kidston (01) p. 46. » Kidston (85). ' White, D. (07"). 



