The _Knorria_ state represents the impression or cast of the outer cortical region too deep below the leaf-cushion region to retain any indications of the cushion-form; the ridges are the casts of the spaces produced in the cortex by the decay of the sheath of delicate cells surrounding each leaf-trace and by the decay of the thin-walled cells of the parichnos. The occasional forked apex of a ridge is the expression of the fact that the cast was made at the region where the parichnos divides into two arms (cf. p. 100). In certain specimens it is possible to connect the Knorria casts with associated lepidodendroid stems which may be determined specifically; but when we have no evidence as to surface-features the fossils may be designated casts of lepidodendroid stems in the Knorria condition. Such casts are illustrated by numerous drawings in palaeobotanical literature[277]. _b._ _Bergeria._ This is another name first used by Sternberg in his classic work, _Die Flora der Vorwelt_, for casts of lepidodendroid plants such as Steinhauer[278] had previously figured as _Phytolithus cancellatus_. Brongniart[279] recognised that the application of the generic title _Lepidodendron_ should be extended to include specimens referred by Sternberg to _Bergeria_, and a few years later Goldenberg[280] realised that this name does not stand for well-defined generic characters. The correctness of these views was, however, first satisfactorily demonstrated by Carruthers[281] and by Feistmantel[282]. If a Lepidodendron stem loses its superficial layers of outer cortex and in this condition is embedded in sand or mud, the cast is distinguished from that of a perfect stem by the absence of the leaf-scars and by other features. It may, however, still show spirally disposed areas, corresponding approximately to the original leaf-cushions, which are characterised by a small depression or pit either at the apex or near the centre of each oval area: the pit marks the position of the leaf-trace and its parichnos strand. In some cases the exposed surface may be smooth without any indication of leaf-cushions, while narrow 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 Potonié[283]. _c._ _Aspidiaria._