452 THE CARBONIFEROUS SYSTEM. 



are particularly rare, and L. undulatum is the most common species. 

 In the Middle Coal formation, L. rimosum, L. dichotomum, L. elegans, 

 and L. Pictoense are probably the most common species ; and L. cor- 

 rugatum is the characteristic Lepidodendron of the Lower Carbonifer- 

 ous, in which plants of this species seem to be more abundant than 

 any other vegetable remains whatever. 



To the natural history of this well-known genus I have little to add, 

 except in relation to the changes which take place in its trunk in the 

 process of growth, and the study of which is important in order to 

 prevent the undue multiplication of species. These are of three kinds. 

 In some species the areoles, at first close together, become, in the pro- 

 cess of the expansion of the stem, separated by intervening spaces of 

 bark in a perfectly regular manner ; so that in old stems, while widely 

 separated, they still retain their arrangement, while in young stems 

 they are quite close to one another. This is the case in L. corrugatum. 

 In other species the leaf-scars or areoles increase in size in the old 

 stems, still retaining their forms and their contiguity to each other. 

 This is the case in L. undulatum, and generally in those Lepidodendra 

 which have very large areoles. In these species the continued vitality 

 of the bark is shown by the occasional production of lateral strobiles 

 on large branches, in the manner of the modem Red Pine of America. 

 In other species the areoles neither increase in size nor become regu- 

 larly separated by growth of the intervening bark ; but in old stems 

 the bark splits into deep furrows, between which may be seen portions 

 of bark still retaining the areoles in their original dimensions and 

 arrangement. This is the case with L. Pictoense. This cracking of 

 the bark no doubt occurs in very old trunks of the first two types, but 

 not at all to the same extent. I figure three examples of these pecu- 

 liarities in mode of growth : — 



Lepidodendron corrugatum, Dawson. — I give below a description 

 of this species, and may refer to the figures in Fig. 168 for further 

 illustration. I do not know any other species in Nova Scotia 

 which has precisely the same habit of growth ; but L. plicatum 

 and L. rimosum show a tendency to it. The present species is 

 exclusively Lower Carboniferous, and occurs on that horizon in New 

 Brunswick, in Pennsylvania, and, I believe, also in Ohio ; though the 

 beds holding it in the latter State have been by some regarded as 

 Devonian. 



L. undulatum, Sternberg. — I think it not improbable that several 

 closely allied species are included under this name. On the other 

 hand, all the large areoled Lepidodendra figured in the books must 

 have branches with small scars, which in the present state of know- 



