80 On the Leaf Scars of Lepidodendron. 



leaf scar can be correctly staty/J only in a general way, as 

 ranging between wide and indefinuje limits. , 



Excessive leanness seems to have been very commonly, if 

 not generally, the condition of these exuberant trees. Nearly 

 all the fossils evidence more or less crowding of the leaf bases. 

 See plates VIII and IX. For this reason the typical form and 

 relation of the scars or foliar cushions must have been some- 

 what uncommon. It requires but slight crowding or mutual 

 compression of the bases of the leaves, to obliterate the keeled 

 line connecting them. This can frequently be seen upon fine 

 specimens with mostly double margins, and is shown in three 

 places in Fig. 2, PI. Y. But in this stage of compression the 

 overlapping ends of adjacent scars will still be separated by a 

 remaining portion of the margin, which has been mostly ob- 

 literated, or rather fused with the contiguous margin. This 

 produces the form originally called L. aculeatum, Sternb. 

 (Figs. 3, 4, PI. V ; Figs. 2, 3, PL VI ; Fig. 1, PI. VIII). In 

 Figs. 1, 2, PI. V, the ends of the leaf scars do not overlap in 

 every instance ; and Figs. 3, 4, PI. V, show the resulting form 

 of compression. But in Figs. 1, 5, PI. VI, the ends overlap 

 considerably ; and Fig. 2, PI. VI, shows the corresponding de- 

 rived form. I use the expression " obliteration by compression," 

 to cover two possible cases. First, a true destruction, by the 

 crowding of the leaves, of certain features which actually ex- 

 isted with the young leaves ; and second, the prevention or 

 suppression of those features by the crowding and mutual 

 pressure of the leaf bases from the time of starting. In speci- 

 mens of full sized leaf scars, it is impossible to distinguish 

 these causes, and in some cases it is likewise impossible to de- 

 termine if the lines connecting the leaf scars have been ob- 

 literated by pressure or destroyed by expansion. Such a case 

 is Fig. 2, PI. VI. But from an examination of the whole 

 specimen, I think it more probable that the connecting lines 

 were obliterated by the coarctate condition of the leaves ; the 

 broad margins being produced by widening of the cortex after 

 the leaves were fully grown. Upon the Plates V and VI, I 

 have indicated the relation of the leaf scars by numerals ; 

 the same figure being applied to the scars that are connected 



