18 Contributions towards establishing the general Character 



affords us two more, which, it will presently be seen^ differ from the others. 

 These latter, however, will vary according to the thickness of either layer 

 of the cuticle ; on those specimens in which this layer is thick, only the 

 vascular scars will be seen ; while on those in which it is thin, there will 

 also be seen, though faintly, an outline of the leaf-scars. The surface of 

 both divisions of the slate, A and B, may agree in this respect, the only 

 difference being, that the scars will be placed on broad furrows in the 

 former, and on ribs in the latter. I have for some time suspected that the 

 two fossils which Brongniart has published under the names of Syringoden- 

 dron pachyderma and Syr. cyclostigma (PI. 166), as well as some figured 

 by Sternberg (PL 13, fig. 1), and Lindley and Hutton {Syr. organum, 

 PI. 70), are true Sigillarice, and that they ought to be classed in the last 

 division, that is, with those specimens which exhibit only the outer sur- 

 face of the innermost layer of the cuticle. The reason for this I con- 

 sider is involved in the fact, that the whole of those fossils shew the vas- 

 cular scars most distinctly on the outer surface of the exposed cuticle, 

 while the leaf scars appear to be absent. Another argument in favour 

 of this view is afforded by Syr. organum just referred to, and some other 

 specimens which have come under my notice : when they are held up 

 to the light in favourable directions, these last shew appearances which 

 resemble the usual form and size of leaf-scars, but which are precisely 

 as if they were caused by impressions that had passed through an inter- 

 vening layer of cuticle from the leaf scars. It requires to be mentioned, 

 however, that I am quite as much in favour of the opinion that the outer 

 surface of the outermost layer of the cuticle is the one exposed on these 

 specimens ; but I cannot agree with Brongniart in supposing that the 

 peculiarity of their scars renders it necessary to place them in a genus 

 distinct from Sigillaria. I am, on the contrary, inclined to think that 

 this peculiarity is merely due to the falling off of the leaves, and the sub- 

 sequent increase of the stem, — two circumstances which, it is evident, 

 would tend to obliterate all traces of the margins of the leaf-scars. Be- 

 sides the scars, the surface of both divisions, as well as that of B, in dia- 

 gram 4, will exhibit a number of longitudinal striae^ which constitute an 

 important character, inasmuch as it enables us to distinguish these sur- 

 faces from the original or external one of the cuticle, which, on the con- 

 trary, is in general marked with transverse lines, either straight, sinuous, 

 or angular.t As the longitudinal strice are always seen on the surfaces 

 resulting from the innermost layer of the cuticle,^ I am inclined to think 



* Vide the decorticated parts of Sigillaria pachyderma (Plate 150, fig. 1) and 

 Sig. Schlotheimi (PI. 152, fig. 4), as figured by Brongniart in his " Vegetaux Fos- 

 siles." 



t Vide Brongniart's figures of Sig. pachyderma and Sig. contracta (Plate 147, 

 fig. 2). 



J The innermost layer of the cuticle of Lepidodendron is also formed of prosen- 

 ehyma, and the surfaces resulting from it exhibit similar longitudinal striae. 

 Casts of the ligneous cylinder of Stigmaria, likewise shew impressions of this 

 ind, which are evidently cauBed by the elongated vessels of which it is formed. 



