SIGILLARIEAE. 



243 



to form ribs is very often shown in the serpentine course of the furrows, 

 which is due to the circumstance that the cushions are broadest at the in- 

 sertion of the leaves and narrow downwards, so that every leaf-scar lies on 

 a knot-like swelling of the rib. This is also accompanied in most cases 

 with a regular alternation of greater or slighter elevation above the surface. 

 An excellent example will be found in Sigillaria contracta l . All possible 

 intermediate cases between these extremes are of course to be seen. The 

 group has received its name from the polygonal leaf-scars, which look like 

 the impressions of seals. These scars are considerably larger than those of 

 Lepidodendrae, and by flattening of the two median angles to a greater or 

 less extent they become more or less decidedly hexagonal, sometimes 

 almost round or ovate. The 

 hexagon will be broader or 

 longer according as the 

 median or lateral boundary- 

 lines are the longer. Dif- 

 ferences of this kind are 

 generally employed to dis- 

 tinguish species. The de- 

 tails of the surface of the 

 scar can usually be seen 

 best in mould-specimens 

 after the remains of the 

 coal have been removed, 

 only rarely in the cast ; they 

 appear in more than usual 

 perfectness when the entire 

 thickness of the rind of coal 

 has separated with them 

 from the mould.. The trace 

 does not lie as in Lepido- 

 dendron on the lower mar- 

 gin of the scar, but in the fe 



middle of the scar or somewhat above the middle. It consists of three small 

 protuberances or impressions, the middle one of which is punctiform or a 

 little elongated transversely and represents the scar of fracture of the vascular 

 bundle. The lateral marks are as a rule elongated into the shape of a 

 stroke or comma, and diverging below deviate from the vertical direction. 

 In some instances, as for example in a stem of Sigillaria elegans from 

 Anzin which lies before me, the deviation may be so great that the marks 

 may become almost horizontal ; usually they enclose the middle trace- 



P> 



FIG. 26. Sigillaria. A surface of Sigillaria Saullii, Brongn. belonging 

 to the group of Rhytidolepis. B surface of Sigillaria hexagona, Brongn. 

 of the section Favularia. C surface of Sigillaria Brardii, Brongn. be- 

 longing to Clathraria. D surface of Sigillaria spinulosa, Brongn. 

 belonging to Leiodermaria. A and B after Brongrjiart (1), C and D 

 after Germar (1). 



1 Brongniart (1), vol. i, t. 147, f. 2. 

 R 2 



