82 On the Leaf Scars of Lepidodendron. 



of the medial line. The other apparent differences will be, con- 

 sidered hereafter. Prof. Schimper observes that L. modulatum 

 greatly resembles L. aculeatum. It could hardly be supposed 

 that modulatum is the typical form ; for it must be understood 

 that the leaf scars are much more likely to be shortened by 

 compression than lengthened. I can imagine no process of 

 modification which could produce the form in Figs. 1, 2, PI. 

 V, from modulatum, or from any form in the plates. The re- 

 sulting difference in form in the production of modulatum is 

 not very striking, but it is the result of an interesting process. 

 The width of the margin bears no fixed relation to the size 

 of the leaf scars. It represents either the space between the 

 cushions of the growing leaves, or the amount of expansion of 

 the cortex after the leaves attain their full growth. It can- 

 not be thought that the leaves upon these rapidly growing 

 plants should always be exactly the same relative distance 

 apart, or the same relative size, — in other words, that the re- 

 lation between the growth of the stem and of the leaves 

 should be absolutely sustained. The rugosity of the margin 

 usually increases with the breadth. But there is much va- 

 riation; some very broad margins are finely wrinkled and 

 plane, while narrow margins are sometimes exceedingly brok- 

 en. The narrow margin is half round and depressed. On 

 old stems, the delicate sculpturesque markings of the scars 

 or cushions become obliterated by loss of the outer cortex; 

 but well preserved specimens are found with the margins 

 as broad as the scars. L. distans, Lesq. (Fig. 6, PL VI) is 

 such an example. Here the leaf scars have precisely the 

 same relation as in Fig. 2, PI. YI. The only feature of L. 

 distans that is in the least degree peculiar is the double ap- 

 pendages. The superior appendages are accessory and non- 

 essential ; they may be seen in Fig. 4, PI. VIII, in a margin- 

 less specimen. I have rarely observed them on single leaf 

 scars. But the same cause which produces a peculiarity on 

 one leaf scar might likewise affect many of the neighboring 

 ones, or all the scars over a large area. I shall hereafter ex- 

 plain the nature of the superior appendages. The size of the 

 leaf scar in distans cannot be regarded as a specific charac- 



