ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 745 
our H. tiliwoides. A second species, H. Bibractense, B.R., has since been described 
by M. Renaurz.* 
Our present detailed knowledge of the structure of Heterangium has been derived 
almost entirely from the study of the English and Scotch specimens. 
As is well known, the genus Heterangium, while closely resembling Lyginodendron 
in habit and many points of structure, is sharply distinguished from it by the primary 
structure of the central cylinder in the stem. In Heterangiwm there is no pith; the 
whole interior of the stele is occupied by the primary wood, consisting of a mass of 
trachez, arranged in groups and interspersed with irregular bands of conjunctive 
parenchyma.t 
Important as this difference is, we find that the agreement in details of structure 
with Lyginodendron is in many points so exact, as to leave no doubt of the close 
affinity of the two genera. The foliage, of which we have some knowledge in the 
case of H. Grievi, bore a general resemblance to that of Lyginodendron. We have 
good reason to believe that the roots were also of the same type of structure as in 
that genus. 
We will begin our description with H. G'rievii, the species of which we have the 
most complete knowledge, though its detailed preservation is seldom equal to that of 
HI. tilieordes. 
i. HeETERANGIUM Grigvi, Will. 
A. THE STEM. 
1. General Structure. 
The stems of Heterangium Grievit, so far as our specimens show, rarely exceeded a 
diameter of 1°5 centim. The smallest stems observed attained about half this 
diameter (see branch in Plate 26, fig. 21) ;~8o the range of dimensions is inconsiderable, 
contrasted with that among the authentic specimens of Lyginodendron. The general 
structure is uniform throughout, the differences depending almost entirely on the 
extent to which secondary growth has taken place. 
There are some slight differences between the Burntisland and the Dulesgate 
specimens, though we have not regarded them as sufficient to necessitate a specific 
separation between these forms. 
The typical structure has been so fully described in former memoirs, that only a 
brief recapitulation is necessary here.{ 
* Renavt, “Structure comparée de quelques tiges de Ja Flore carbonifére,” 1879, pp. 276-278, 
Plate 14, figs. 4-8. Berrranp and Renavy, “ Recherches sur les Porowylons,” ‘Archives bot. du Nord 
de la France,’ 1886, p. 245. Rewnautt, ‘Flore fossile d’Autun, &c.,” Part II., Atlas, Plate 65, 1894. 
+ See Witzramson, ‘ Organization,” Part IV., Plate 28, fig. 30; Part XIII., Plate 21, fig.1; Plate 22, 
fig. 2; also our Plate 28, fig. 30. 
+ Witiiamson, “ Organization,” Parts IV. and XVII. 
MDCCCXCV.—B. 5 D 
