1 96 LEPIDODENDREAE. 



form, and in very many cases through the opening out of the lower angle 

 of the rhomb it approximates to an equilateral triangle with one angle at 

 the upper end. In normal circumstances its lower corner nearly coincides 

 with the point of intersection of the diagonals of the rhombic cushion or 

 lies a little above them. On its flat surface near the lower margin three 

 marks are seen, which in casts are depressed, in moulds, where they are 

 usually very conspicuous, are strongly protuberant. They may be all of 

 the same shape and dot-like, but in some cases the lateral ones are elongate 

 and linear, and the middle one only is either punctiform, or triangular with 

 one angle at the lower end and the sides prolonged and in the shape of a 

 V. Nothing is more natural than to see in these three marks the points of 

 severance of the leaf-trace bundles, and this is the view of most authors. 

 But since we know from Renault's l observations on analogous conditions 

 in Sigillariae that in these plants only the middle mark really answers to 

 the transverse fracture of a vascular bundle, while the lateral marks repre- 

 sent a different point of structure, it will be well to assume provisionally 

 that this is the case also in the group before us ; but the matter will have 

 to be considered again more fully in dealing with Sigillaria. At present we 

 have in fact nothing to go upon but such a conclusion from analogy, 

 because examination into the anatomy of Lepidodendrae has as yet thrown 

 no light on the question. 



Both the area of the scar and the outline of the leaf-cushion represent 

 transverse sections of a four-sided pyramid, and the angles of both are 

 therefore connected by projecting edges which divide the surface of the 

 cushion into four laterally disposed segments, the faces of the pyramid. The 

 median ridges running in a straight line are more strongly marked than the 

 lateral ones ; the lower one is generally longer than the upper, because the 

 scar has been moved a little out of its position in the upward direction. 

 They bisect the upper and the lower acute angle of the rhomb. Each of 

 the lateral edges on the other hand, running a little obliquely downwards 

 from the lateral angles of the scar, divides the lateral angles of the rhombic 

 cushion into an upper more acute and a lower less acute angle. In well- 

 preserved specimens the facets may show a delicate velvety punctation of 

 their entire surface, caused probably by the impression of the individual 

 epidermis-cells ; in some species they are marked with transverse folds and 

 wrinkles, which are sometimes very prominent. Again, in the upper angle 

 of each of the two lower and larger facets, and near the median ridges 

 which separate them, there is usually a conspicuous roundish or ovate 

 depression, which may however sometimes appear as a projection. Stur 

 proposes to call both of these marks the vascular glands of the leaf-cushion, 



Renault (8). 



