THE CARBONIFEROUS PLORA. 157 



ately prominent. Leaf-soars rhombic or sometimes shield-shaped or 

 heart-shaped, in the middle or upper part at the leaf-base. Vascular 

 scars three — the middle one always largest and corresponding to the 

 single nerve of the leaf ; the lateral ones spmetimes obsolete. 



In older stems three modes of growth are observed. In some 

 species the expansion of the bark obliterates the leaf-bases and 

 causes the leaf-scars to appear separated by wide spaces of more or 

 less wrinkled, bark, which at length becomes longitudinally furrowed 

 and simulates the ribbed character of SigiUaria. In others the leaf- 

 basra grow in size as the trunk expands, so that even in large trunks 

 they are contiguous though much larger than those on the branches. 

 In others the outer bark, hardening at an early age, is incapable of 

 either of the above changes, and merely becomes cleft into deep fur- 

 rows in the old trunks. 



Lepidophloios. — Leaf -bases transverse and prominent — often 

 very much so. Leaf-scars transversely rhombic or oval with three 

 vascular scars, the central largest. Leaves very long and one- 

 nerved. Large strobiles or branchlets borne in two ranks or spirally 

 on the sides of the stem, and leaving large, round scars (cone-scars), 

 often with radiating impressions of the basal row of scales. 



Species with long or drooping leaf -bases have been included in 

 Lepidophloios and Lomatophloios. Species with short leaf -bases and 

 cone-scars in two rows have been called TJlodendron, and some of 

 them have been included in SigiUaria (sub-genus Claihraria). De- 

 corticated stems are Bothrodendron and Halonia. Some of the 

 species approach near to the last genus, especially to the Lepidoden- 

 dra vrith rhombic leaf -bases like L. tetragonvm. 



Cyclostigma. — Leaf -bases undeveloped. Leaf -scars circular or 

 horseshoe-shaped, small, with a central vascular scar. In old trunks 

 of Cyclostigma the leaf-scars become widely separated, and some- 

 times appear in vertical rows. Young branches of Lepidodendron 

 sometimes have the leaf-scars similar to those of Cyclostigma. 



LeptophUum. — Leaf-bases flat, rhombic; leaf-scars obsolete; 

 vascular scar single, central. The last two genera are character- 

 istically Devonian. 



In contradistinction from the trees above mentioned, the follow- 

 ing general statements may be made respecting other groups : 



In conifers the leaf-bases are usually elongated vertically, often 

 scaly in appearance, and with the leaf -scar terminal and round, oval, 

 or rhombic, and with a single well-marked vascular scar. 



In Calamites, Calamodendron, and Asterophyllites the scars of 

 the branchlets or leaves are circular or oval, with only a single vas- 



