XXXIl] CLADOXYLON 201 



Unger as stems and does not agree with the inclusion of any of 

 them in the Rhachiopterideae. Without losing sight of the fact 

 that Bertrand's conclusion is not based on proof but is the 

 expression of a view suggested by a close agreement in general 

 anatomical plan, I venture to adopt the designation Cladoxylon 

 in a wide sense primarily on the ground that Bertrand's view is 

 probably correct and in part for the sake of convenience of 

 description. As Unger's species of Cladoxylon differ from one 

 another in features which may fairly be regarded as of minor 

 importance, they are included under one specific name. 



Cladoxylon. Unger. 



Cladoxylon mirabile Unger 1 . The following are regarded as 

 specifically identical with or closely allied to Cladoxylon mirabile : 

 C. dubium, Schizoxylon taeniatum, Hierogramma mysticum, Syn- 

 cardia pusilla, Arctopodium insigne and A. radiatum 2 . 



i. Stems. The stems assigned to Cladoxylon are characterised 

 by a complex system of steles, either simple or branched and 

 occasionally anastomosing, presenting in transverse section the 

 form of oval or cylindrical strands or narrow, straight or curved 

 bands arranged on a more or less clearly marked radial plan 

 (fig. 459, A, B, D). In some stems the primary vascular tissue 

 is enclosed by secondary xylem and phloem (fig. 460, B), while 

 in others (Unger's Arctopodium, Hierogramma 3 , Syncardia) there 

 is no evidence of secondary thickening. The diagrammatic 

 drawing represented in fig. 459, F, shows a section of a small 

 axis, regarded by Unger and Solms as a petiole (3 mm. in diameter), 

 containing four vascular strands composed exclusively of primary 

 xylem, each with one or, in the case of a double strand, two 

 protoxylem groups. This type may be a slender stem or branch 

 or possibly a petiole. The other extreme, as regards complexity 

 of vascular structure, is represented by such stems as those shown 

 in fig. 459, A, B, D. In Unger's Cladoxylon mirabile (fig. 459, A; 

 fig. 460, B) the stem reaches a diameter of 3 cm. and consists 

 of several radially disposed plates of vascular tissue with an 



1 Unger and Richter (56) B. p. 179, PL xn. figs. 6, 7; Solms-Laubach (96) B. 

 p. 52, PL n. figs. 11, 13(?). 



2 Unger and Richter, PL xn. figs. 3, 4; Solms-Laubach (96) B. PL xi. fig. 10. 



3 See footnote 3, p. 205. 



