414 STUDIES IN FOSSIL BOTANY 



the other tissues, agrees in structure with that of the 

 former species. 



The outgoing leaf-trace bundles are nearly always 

 found in pairs, as in Lyginodendron. Judging from the 

 one instance in which a petiole has been found, in this 

 species, in connection with the stem, it appears that 

 the two bundles united on entering the leaf-base. 



In H. tiliaeoides ; as in the other forms, adventitious 

 roots were borne on the stem, but we are not as yet 

 acquainted with their detailed structure. 



Apart from the exceptionally perfect preservation of 

 its tissues, Heterangium tiliaeoides is of special interest 

 from the fact that in several details — as in the distinct- 

 ness of the peripheral xylem-strands of the stele, the 

 distribution of the sclerotic elements, and the paired 

 leaf-traces — it approaches Lyginodendron oldhamium, 

 while in its general structure it retains all the essential 

 Heterangium characters. The affinity of this species 

 with Lyginodendron is so evident as to prove beyond 

 reasonable doubt that the proper place of Heterangium 

 is in the Lyginodendreae, rather than in the Medulloseae 

 (see next chapter). It is also interesting to see how 

 closely the detailed structure of the secondary tissues, 

 in this Fern-like plant, resembled that in the recent 

 Cycads. 



5. Habit. — As regards the external habit of the 

 genus Heterangium, we have good evidence in the case 

 of the species H. Grievii. From the study of the 

 specimens with structure preserved, we know that the 

 plant possessed long angular stems, about a centimetre 

 or a centimetre and a half in diameter, bearing, at 



