INTRODUCTION. 7 



to be mentioned may add considerably to the complications. Both mould 

 and cast in the same object, for example in a stem, may turn out quite 

 differently according to the condition in which it was inclosed in the 

 petrifying substance, a fact to which Steinhauer 1 , as far as I know, was 

 the first to call attention. For example, every one is acquainted with the 

 ordinary impressions of Sigillaria, such as are common on every heap of 

 refuse from our coal-pits, which answer to the outer surface of the epidermis 

 and are characterised by the projecting rib-like orthostichies with the leaf- 



FIG. i. Dictyoxylon-structure of the rind as it occurs in some Lepidodendraeand Sigillariae, and in Lyginodendron, 

 &c. A transverse section of the outer rind of Lepidodendron rhodumnense, B. Ren., showing the anastomosing plates 

 of sclerenchyma which bound the meshes filled with parenchyma ; the longitudinal section is quite similar, only the 

 sclerenchymatous elements are seen in elongated form. B impression of the inner side of such a Dictyoxylon-rind 

 separated from the stem. The ribs are more prominent owing to the disappearance of the parenchyma, and answer to 

 the furrows. The rhombic cushions fill the depressions caused by this disappearance of tissue in the meshes. A after 

 Renault 2 , B after Williamson 3 . 



scars marked on them at regular distances. The examination of silicified 

 specimens shows that the parenchyma of the rind of these plants was 

 traversed by a net-work of sclerenchymatous strands forming vertical 

 elongated meshes of irregular rhombic form (Fig. \ A). Now impressions 

 exactly reflecting this structure are found, though not too frequently, in 

 which the rhombic areolae appear as similar convex projections of unequal 

 elevation (Fig. I B}. There are many reasons for believing that in these 

 objects, which look so unlike one another, we have merely the impressions 

 of stems of Sigillarias stripped of their rind, and with the net-work of 



Steinhauer (1). 



2 Renault (1). 



3 Williamson (1), iv. 



