XV] ASPIDIARIA 127 



spirally arranged grooves represent the obliquely ascending 

 vascular bundles passing through the cortex to the leaves. 



Fig. 185, B, shows the Bergeria state of Lepidodendron 

 Veltheimianum, which differs from the Knorria condition in the 

 fact that decortication had not extended below the level at 

 which the form of the leaf-cushions could be recognised. It is 

 clear that no sharp line can be drawn in all cases between the 

 different degrees of decortication as expressed by the terms 

 Knorria and Bergeria. 



A list of synonyms of Knorria, Bergeria, and Aspidiaria 

 forms of stem and a detailed treatment of their characteristic 

 features may be found in a recent work by Potoni^\ 



c. Aspidiaria. 



In one of the earliest English books on fossil plants, the 

 Antediluvian Phytology by Artis^ a specimen from the Car- 

 boniferous sandstone of Yorkshire is figured as Aphyllum 

 cristatum, and a similar fossil is described as A. asperuTn. 

 These are impressions of Lepidodendron stems in which the 

 characteristic leaf-cushions are replaced by smooth and slightly 

 convex areas with a narrow central ridge. To this type of 

 specimen Presl gave the name Aspidiaria^, under the impres- 

 sion, shared by subsequent writers, that the supposed external 

 features were entitled to generic recognition. 



It is to Stur* that we owe the first satisfactory interpretation 

 of fossils included under the name Aspidiaria : he showed that 

 on the removal of the projecting convex areas from some of his 

 specimens a tjrpical Lepidodend^^on leaf-cushion was exposed 

 (fig. 144, A, a). The Aspidiaria condition (fig. 144, A, 6) repre- 

 sents the inner face of the detached shell of outer bark of a 

 Lepidodendron stem, while in the Bergeria casts we have a 

 view of the external face of a stem deprived of its superficial 

 tissues. 



In a Lepidodendron stem embedded in sediment the more 

 delicate portions of the leaf-cushions would tend to shrink away 



1 Potonie (05) Lief. iii. 42—44. " Artis (25) A. Pis. xvi. xxiii. 



3 Sternberg (38) A. * Stur (75) A. Heft ii. p. 229. 



