The lower surface of the leaf, seen in section in fig. 465, would show a number of approximately equal ribs, or possibly primary ribs (midway between the veins) separated by two interstitial ribs, while on the upper face there would be three rather smaller secondary ribs. In a section of a leaf called by Renault _C. crassus_[561], a specific name used also by Lesquereux[562] for an impression of a leaf originally described by Goeppert as _Noeggerathia crassa_, there are deep stereome-strands between the veins next the lower epidermis alternating with single smaller strands, while on the upper surface the hypodermal strands occur only immediately above the veins. In a section figured by Felix[563] from North Germany as _C. robustus_, the hypodermal stereome forms continuous bands; on the upper face the bands are uniform in thickness but next the lower epidermis they form a series of ribs. Grand’Eury’s subgeneric terms _Cordaites_, _Dory-Cordaites_, and _Poa-Cordaites_ have therefore very little value as regards differences in the ribbing of leaf-impressions: the large size of leaves included in _Dory-Cordaites_ and the more acute apex of the lamina as compared with the obtuse apex of smaller leaves of _Cordaites_ are features of limited application and of minor importance as diagnostic characters. The name _Poa-Cordaites_ is, however, usefully employed for the narrower linear leaves with an obtuse apex. The structure of a _Cordaites_ leaf is clearly shown in fig. 465; the lamina is approximately 1 mm. thick and there are about 30 veins in a breadth of 2 cm. Strong I-shaped girders with the webbing composed of thick-walled cells divide the mesophyll into rectangular compartments: the intervening hypodermal strands differ in number and size on the two faces. The epidermis is not preserved: specimens of other leaves show that the stomata[564] occur in rows on the lower surface. The mesophyll shows no differentiation into palisade and spongy parenchyma, and in this respect the leaf agrees with many other forms; but in some leaves the palisade-tissue is well developed, as in _C. lingulatus_ Ren.[565] The central region of the lamina consisted of lacunar tissue, portions of which are preserved, with a more compact sheath of parenchyma enclosing each vein. In some leaves there is a narrower sheath of thick-walled cells more sharply contrasted with the mesophyll. The vascular bundles agree in structure with those in the rachis of a Cycadean frond more closely than with the veins of an Araucarian or other Coniferous leaf. The xylem consists mainly of centripetal elements which form a deltoid strand with the protoxylem at the apex, and in close association with this is a larger or smaller amount of narrower centrifugal tracheids: in the section shown in fig. 465 the centrifugal xylem may extend all round the centripetal tracheids, but it usually forms an irregular arch with its base attached to the