156 THE GEOLOGICAL HISTORY OF PLANTS. 



mens of this kind, taken alone, have been referred to the genera 

 Knorria, Bothrodendron, and Halonia, 



4. In some cases, though not frequently, the outer surface of the 

 ligneous cylinder is preserved. It almost invariably presents a 

 regularly striated or irregularly wrinkled appearance, depending 

 upon the vertical woody wedges, or the positions of the medullary 

 rays or vascular bundles. Specimens of this kind constituted some 

 of the Endogenites of the older botanists, and the genus Schizoden- 

 dron of Eichwald appears to include some of them. Many of them 

 have also been incorrectly referred to Calamites. 



5. In some cases the cast of the medullary cylinder or pith may 

 alone be preserved. This may be nearly smooch or slightly marked 

 by vertical striae, but more usually presents a transverse striation, 

 and not infrequently the transverse constrictions and septa charac- 

 teristic of the genus Sternbergia. Loose Sternbergice afford little 

 means of connecting them with the species to which they belong, 

 except by the microscopic examination of the shreds of the ligneous 

 cylinder which often cling to them.* 



These facts being premised, the following general statements 

 may be made respecting some of the more common Palaeozoic genera, 

 referring, however, principally to the perfect markings as seen on 

 the epidermis : 



Sigillaria. — Leaf-bases hexagonal or elongated, or confluent on 

 a vertical ridge. Leaf-scars hexagonal or shield-shaped. Vascular 

 scars three, the two lateral larger than the central. This last char- 

 acter is constant, depending on the fact that the leaves of Sigillaria 

 have two or more vascular bundles. All so-called Sigillarice having 

 the central vascular scar largest, or only one vascular bundle, should 

 be rejected from this genus. In young branches of branching Sigil- 

 larice the leaf-scars sometimes appear to be spiral, but in the older 

 stems they form vertical rows ; interrupted, however, by transverse 

 rows or bands of fruit-scars, each with a single large central vascular 

 scar, and which have borne the organs of fructification. Arthro- 

 caulis of McCoy is founded on this peculiarity. 



Syringodmdron. — Differs from Sigillaria in the leaf-scars, which 

 are circular and with a single vascular bundle. It is a matter of 

 doubt whether these plants were of higher rank than Sigillaria 

 tending toward the pines, or of lower rank tending toward Cyclo- 

 stigma. Their leaf-bases form vertical ridges. 



Lepidodendron. — Leaf-bases rhombic, oval, or lanceolate, moder- 



* See my paper, " Journal of Geological Society," vol. xxvii. 



