96 LYCOPODIALES [CH. 



features characteristic of Lepidodendron stems it is essential to 

 have some knowledge of the internal structure. 



A dual system of terminology has been unavoidably adopted 

 for species oi Lepidodendron: the majority of specific names have 

 been assigned to fossils known only in the form of casts or im- 

 pressions, while petrified fragments, which unfortunately seldom 

 show the surface-features, have received another set of names. 

 A glance at the older palaeobotanical literature reveals the 

 existence of several generic designations, which fuller information 

 has shown to have been applied to lepidodendroid shoots 

 deprived of some of their superficial tissues before fossilisation 

 and differing considerably in appearance from the more complete 

 branches of the same species ^ It has in some instances been 

 possible to correlate the two sets of specimens, casts or im- 

 pressions, showing external features, and petrified fragments. 

 We may reasonably expect that future discoveries will enable 

 us to piece together as definite specific types specimens at 

 present la,belled with different names. 



A well-preserved leaf-cushion of a Lepidodendron — the most 

 obvious distinguishing feature of the genus — is rhomboidal or 

 fusiform and vertically elongated (fig. 146, C, E; fig. 185, C, D): 

 in exceptional cases it may reach a length of 8 cm. and a breadth 

 of 2 cm. The cushion as a whole represents a prominent portion 

 of the stem or branch comparable with the elevation on the twig 

 of a Spruce Fir and the leaf-base of a Lycopodium (cf. fig. 121, A, 

 lower portion) which appears in a transverse section of a branch 

 as a rounded prominence (cf. Lycopodium, fig. 125, A and H). 

 Disregarding diflferences in detail, a typical Lepidodendron 

 leaf-cushion is characterised by a clearly defined smooth 

 area often situated in the middle region (fig. 146, C, s). 

 This is the leaf-scar or place of attachment of the base of 

 the leaf which was cut off by an absciss-layer while the branch 

 was comparatively young, as in recent forest trees and in some 

 species of Ferns. On the leaf-scar are three smaller scars or 

 cicatricules, the central one is circular or more or less triangular 

 in outline, the two lateral scars being usually oval or circular. 

 The central pit marks the position of the single vascular bundle 



' See Fischer (04). 



