226 CORDAITEAE [CH. 



anatomical characters as criteria by which to distribute the foliage 

 of the Cordaites form among the genera Cordaites, Mesoxylon, 

 and Poroxylon, using these names as designations of certain types 

 of anatomical structure. The specimen reproduced in fig. 465 

 is in all probability a piece of a leaf of Cordaites principalis, but 

 on anatomical grounds Miss Benson 1 has made it the type of a 

 new species, C. Felicis, and more recently Scott 2 has brought 

 forward evidence supporting the view that it is a leaf of Mesoxylon. 

 As, therefore, neither impressions nor petrifications of Cordaitean 

 leaves can in the great majority of cases be referred with confidence 

 to their respective genera of stems, pending fuller information 

 the only course would seem to be to use the name Cordaites in 

 a comprehensive sense indicating in special cases where evidence 

 is available the more precise systematic position of the specimen. 

 The classification of Cordaitean leaves proposed by Grand' Eury 3 

 is based partly on the form of the lamina and in part on the equality 

 or inequality of the ' veins.' The actual veins, which are embedded 

 in the fairly thick mesophyll, do not directly affect the super- 

 ficial ribbing on the carbonised impression of the leaves and, as 

 seen in fig. 465, the most prominent hypodermal strands of sup- 

 porting tissue which would appear as the main veins or primary 

 ribs on an impression do not correspond in position with the 

 vascular bundles. Although in some cases the largest stereome- 

 strands coincide with the veins, forming the upper and lower 

 parts of I-shaped girders the centre of which is occupied by the 

 veins, this is by no means always the case. Grand' Eury has 

 drawn attention to the difference between the upper and lower 

 surface of some carbonised leaves : in C. crassifolius (fig. 468, D) 4 

 there are five to seven finer ribs between each pair of primary 

 ribs on one face while the other shows ridges and grooves with 

 a rib corresponding to each. Attention is called on a later page 

 to the variable character of the ribbing even on different parts of 

 the same lamina. 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 



1 Benson (12). 2 Scott (12) p. 1022. 



3 Grand'Eury (77) A. p. 208. 4 Grand'Eury (90) A. p. 321. 



