Knowledge of Lyginodendron. 83 
manner as to closely resemble the appearance presented by 
some of the sections of Lyginodendron robustum. Williamson 
and Scott do not attach the slightest importance to the 
anomalous medullary cambium as a specific character ; they 
point out that ‘among stems, which are perfectly similar in 
other respects, some show it and some do not, while in those 
that possess this anomaly, the degree in which it is developed 
is most variable V 
Numerous sclerous nests and secretory sacs occur in the 
pith of Lyginodendron robustum , and the latter are also 
present between the centrifugal and centripetal wood, and 
in the medullary-ray-tissue. An extended examination of 
several sections leads to the conclusion that Williamson and 
Scott’s opinion that Nield’s specimen ‘ really belonged to 
a Lyginodendron , or to some plant of the same type of 
structure,’ is no doubt correct. Lyginodendron robustum 
represents by far the largest stem of the Lyginodendron type 
so far recorded, and the considerable thickness of the second- 
ary xylem renders more obvious the close correspondence 
with the wood of recent cycadean stems, than is apparent 
in the much smaller stems of Lyginodendron Oldhamium . 
In certain genera of fossil plants, one of the most prominent 
characteristics is the very close resemblance of their centri- 
fugal xylem to the wood of recent Cycads. Lyginodendron , 
Heterangium and Cycadoxylon afford three examples of such 
agreement, and other genera might be mentioned. It is not 
unusual to discover specimens of plant-fragments in which 
the secondary xylem-characters are clearly preserved, but 
from which it is impossible to formulate a complete diagnosis. 
It is often impossible to rely on the structure of the 
secondary wood alone as a means of specific or even generic 
identification ; but the nature of the xylem-elements, the 
1 Loc. cit., p. 723. Examples of Lyginodendron Oldhamium showing an 
unequal development of centripetal wood are figured by Williamson (Mem. XVII. 
Phil. Trans., 1890, Plate XIII, Fig. 3) and by Williamson and Scott (Phil. Trans., 
1895, Plate XXIII, Fig. 8). Both sections were cut from the same specimen; 
also sections 1142 and 1885 E. 
G 2 
