The Fossil Plants of t/ie Coal Measures. 97 



to facilitate references to certain types of organisation, some 

 of which may ultimately prove to have a specific value, if 

 we ever succeed in identifying their respective reproductive 

 organs ; but meanwhile many of those groups named most 

 probably embrace several distinct though closely allied 

 species. On the other hand, intermediate examples are 

 already known, and more will doubtless be discovered, 

 tending to unite these separate groups. This latter fact 

 makes it difficult to define clearly the characteristics of each 

 of these groups ; hence my definitions, like my names, 

 can only be regarded as provisional. Meanwhile I have 

 endeavoured to indicate, in as concise a manner as I am 

 able, the more prominent features of each group. 



Type of Lepidodendron Selaginoides Sternberg ? Lepidoden- 

 dron vasculare, Binney ; Sigillaria vascularis, Binney ; 

 Lepidodendron Selaginoides, Camtthers. 



Sections of the youngest twigs, including the leaves, 

 rather thick, -45 of an inch in diameter. Primary central 

 strand of long isomerous barred Tracheae,* intermingled in 

 its centre with numerous barred medullary cells. Innermost 

 Cortex (liber or cambium?) of small celled parenchyma, con- 

 taining numerous transversely intersected leaf-traces. Second 

 cortical zone of larger and more uniform cells, with thicker 

 walls. More externally, a zone of still larger cells, bounded 

 peripherally by a prosenchymatous zone ; very thin in the 

 youngest twigs, but increasing to a considerable thickness 

 as growth advances. 



At a very early period an exogenous secondary xylem 

 strand makes its appearance, commencing at some minute 



* For a good illustration of these Trachese see the longitudinal section, 

 Cabinet No. 376A. I have recently discovered that in the most perfectly pre- 

 served examples the transverse bars of these tracheae are connected by extremely 

 delicate paralled vertical threads, like those described on p. 107, as characteristic 

 of L. Wiinschiaiinni, 



