84 On the Leaf Scars of Lepidodendron. 



to me that L. giganteum lacks several features that are com- 

 mon to the different forms of L. aculeatum. According' to my 

 observations thus far, it is identical with L. clypeatum, Lesq., 

 and L. tetragonum, Sternb. But L. tetragonum may also be de- 

 rived, I think, from L. aculeatum. 



Fig. 2, PI. VII, illustrates a specimen of marginless 

 aculeatum with the leaf scars rounded above ; and as the in- 

 ferior ends of the same are acuminate, there are left triangular 

 spaces of the cortex, which were not covered by the leaf bases. 

 The superior ends of the leaf scars in Fig. 4 are rounded or 

 arcuate, but the triangular spaces of Fig. 2 are in this specimen 

 incorporated with the inferior ends of the leaf scars, making 

 them fish-tailed in shape. 



Upon plates VIII and IX, I have figured several remark- 

 ably compressed forms. Associated with those represented in 

 Figs. 3, 5, 6, PI. VIII, and Figs. 1, 3, 5, 7, PL IX, were found 

 many specimens proving the derivation of these forms from 

 L. aculeatum, and also the transition from that through these 

 forms to L. carinatum, Lesq. (Fig. 2, PI. IX). A careful study of 

 these figures will show that the changes in the relative lengths 

 of the corresponding sides of the leaf scars in the several 

 specimens, are not due so largely to variation in the relative 

 positions of the leaf scars (produced, one may imagine, by 

 sliding the spiral ranks upon each other), as to mutual com- 

 pression and consequent shortening of their overlapping ends. 

 For example, the shortening of the ends in Fig. 1, PI. VIII, 

 first produces the four-sided leaf scar of Fig. 5, PI. IX. 

 Further crowding shortens the ends of the quadrilateral leaf 

 scars still more, bringing other naturally separated scars into 

 contact, and making them six-sided again, as in carinatum and 

 all the remaining figures on PI. IX. This latter change can 

 be seen in Fig. 3, PI. IX, passing from the right side to the 

 left. Now, we have a third spiral of contiguous scars, and a 

 relation of the scars the same as in modulatum. Indeed, it is 

 young modulatum, of the kind having the line connecting the 

 leaf scars an extension of the medial line; or it may, if the 

 stem grows proportionately faster than the leaves, produce a 

 broad typical aculeatum. The relation of the leaf scars in 



