On the Fossil Plants of the Coal-measures. 135 



Externally to this bast-layer is a more superficial epiderm of paren- 

 chyma, supporting the bases of leaves, which consist of similar 

 parenchymatous tissue. Tangential sections of these outer cortical 

 tissues show that the so-called " decorticated " specimens of Lepi- 

 dodendra and of other allied plants are merely examples that have 

 lost their epidermal layer or had it converted into coal, this layer, 

 strengthened by the bast-tissue of its inner surface, having remained 

 as a hollow cylinder when all the more internal structures had been 

 destroyed or removed. 



From this type the author proceeds upwards through a series of 

 examples in which the vessels of the medulla become separated from 

 its central cellular portions and retreat towards its periphery, forming 

 an outer cylinder of medullary vessels, which are arranged without 

 order and enclose a defined cellular axis ; at the same time the en- 

 circling ligneous zone of radiating vessels becomes yet more deve- 

 loped, both in the number of its vessels and in the diameter of the 

 cylinder relatively to that of the entire stem. As these changes are 

 produced, the medullary rays separating the laminae of the woody 

 wedges become more definite, some of them assuming a more com- 

 posite structure, and the entire organization gradually assuming a 

 more exogenous type ; at the same time the cortical portions retain 

 all the essential features of the Lepidodendroid plants. Commen- 

 cing with the Lepidodendron selaffinoides just described, we pass on 

 to L. Harcourtii, in which there is a distinct cellular axis to the 

 medulla, surrounded by a ring of medullary vessels, external to 

 which is the second or radiating cylinder of vessels, from which alone, 

 as M. Brongniart has very correctly shown, the bundles of vessels 

 supplying the leaves are derived. Then we reach the more highly 

 organized of the forms which Mr. Binney has described under the 

 common name of Sigillaria vascularis, in which the woody cylinder 

 is more extensively developed. This conducts us to a series of 

 varieties from which the cells of the medulla have disappeared, but 

 in which there is a very distinct inner cylinder of large barred 

 vessels not arranged in radiating order, and an outer and much more 

 ample cylinder of smaller ones arranged on the exogenous type. In 

 these examples the line of demarcation between the vessels of the 

 medulla and those of the ligneous zone is sometimes straight, and 

 at others boldly crenulated. In the latter examples the outside of 

 the vascular medullary cylinder, detached from its surroundings, ex- 

 hibits the fluted appearance of a Calamite, for which it might be 

 mistaken, but it lacks the transverse nodal constrictions of that genus. 

 It is to some of these more highly organized Lepidodendra just re- 

 ferred to that Corda has applied the name of Biploxylon, and Witham 

 that of Anabathra, both of which correspond in the closest manner 

 with the Sigillaria elegans of M. Brongniart. We are thus brought, 

 by the evidence of internal organization, to the conclusion that the 

 plants which Brongniart has divided into two distinct groups, the 

 one of which he has placed amongst the vascular Cryptogams, and 

 the other amongst tbe Gymnospermous Exogens, constitute one great 

 natural family. 



