XXIX] HETBEANGIUM 83 



in the pericycle. The peripheral leaf-traces in the stele show the 

 mesarch -structure rather more distinctly than in H. Grievii, and 

 the secondary xylem, which forms a much broader cylinder 

 than in the Scotch type, is divided by broad medullary rays 

 into characteristic cuneate masses each of which rests at its base 

 on the centrifugal tracheids of a leaf-trace strand of xylem 

 {fig. 415, B). The most striking distinctive feature is afforded 

 by the secondary phloem, which is often preserved in wonderful 

 perfection; this is unusually thick and owing to the tangential 

 •expansion of the principal medullary rays the secondary phloem 

 is divided into separate masses which decrease in breadth towards 

 the external arcs of primary phloem. The triangular form of 

 the phloem rays, composed of tangentially stretched parenchyma, 

 suggested the specific name tiliaeoides on account of their striking 

 resemblance to the rays of Tilia. The leaf-traces are nearly 

 always in pairs as they pass out through the cortex ; they sub- 

 sequently divide and appear as four vascular strands in the 

 petiole. The portion of stem reproduced in fig. 415, B, 8 mm. 

 broad, shows clearly the separation of the secondary xylem and 

 phloem into wedge-shaped groups : in each group there are several 

 narrow medullary rays. The extrastelar tissues are represented 

 by a few fragments only. Several layers of crushed periderm 

 occur in the pericyclic region but the more external tissues have 

 been almost completely exfoHated^ 



Reference has already been made to Heterangium Lomaxi, 

 the English type originally included by Williamson in Heterangium 

 Grievii. The provisional species Heterangium cylindricum William- 

 son and Scott ^ differs, as Scott says, in no important respect from 

 H. Lomaxi and should not be retained. A new species, H. mini- 

 mum Scott^, has been founded on a very small stem from the 

 •Coal Measures of Dulesgate in which the leaf-traces leave the 

 .stele as single bundles as in the Scotch H. Grievii. 



The French species Heterangium Duchartrei^ Ren. from Permian 

 rocks was originally referred by Renault to the genus Poroxylon : 

 it is represented by little more than the xylem of the stele and 



^ For a fuller description, see Williamson and Soott (95). 

 2 Scott (09) B. p. 410; (15). » Scott (15). 



" Eenault (79) B. p. 276, PI. xiv. figs. 4^8; (96) A. p. 251, Pl.LXV. figs. 1, 2. 



6—2 



