CYCADOFILICES FROM THE CALCIFEROUS SANDSTONE SERIES. 823 
periphery than in others, and that secretory sacs occur in #. tulixoides in the medullary 
rays and in the inner cortex. One characteristic feature of Heterangvwm—the sclerotic 
dises in the inner cortex—finds no parallel in Rhetinangium, while the peculiar outer 
cortex of the latter is not seen in the former. 
As far as the petiole-trace is concerned the two types are distinct, yet there seems to 
be some connection between them. In Pl. III. fig. 21 an exceedingly well-preserved 
petiole-trace of Heterangiwm grievi is shown; it is sub-triangular in shape, and the 
protoxylem is mesarch (PI. III. fig. 21, pra.). The petiole-trace in Rhetinangium, as 
we have seen, is much more complex and it is exarch, yet it looks, to some extent, like 
an ageregation of several traces, each similar to a Heterangiwm trace in which the 
base of the triangular xylem has disappeared. Now, in Heterangium the petiole-trace 
gradually assumes a U shape as it is followed outwards, though it has always mesarch 
protoxylem in two groups, one in each arm of the bilobed (U-shaped) trace. 
Relationships with the more complex pteridosperms are not very evident, but the 
peculiar form of the undivided petiole-trace appears to indicate a transition from the 
simple type so characteristic of the Lygunodendrex to the much divided type in the 
Medullosex. ‘The outer cortex of the stem and petioles, in transverse section, is partly 
of the Myeloxylon landriotw and partly of the M. radiata type, both of which are 
common in the Medullosex though not confined to that group. But in longitudinal 
section the sclerotic fibres are occasionally seen to join, thus forming a reticulum. The 
exarch xylem of Rhetinangium suggests aftinities also with Sutcleffia and Stenomyelon. 
The nearest relative seems to be Megaloxylon, though that genus is more highly 
specialised in at least one direction. The difference between the two seems exactly 
parallel to what occurs among the Osmundacewx, and the explanation that the short 
tracheide is probably a specialisation for water-storage has also been adopted by Kinston 
and GwyNNE-VAUGHAN in their memoirs on the Fossil Osmundacex.* 
Taking the Cycadofilices as a whole, the more ancient types show a simpler structure, 
and, since Rhetinangium occurs at a lower horizon than Megaloxylon, we would expect 
it to show a relatively simpler organisation. The primary wood in the former genus 
contains long tracheides and is much smaller in diameter than is the ease in Megaloxylon, 
and it may be argued that the inner tracheides in the latter stem would be too far from 
the periphery to perform the function of water conduction except near the apex of the 
stem. They therefore appear to have terminated their growth and become modified for 
water-storage, whereas those in Rhetinangium retain their primary function throughout. 
In concluding this paper I desire to express my thanks to Dr Arper, Dr Krpsron, 
F.R.S., and Dr Scorr, F.R.S., for their unfailing kindness and valuable suggestions 
whenever difficulties arose, and also for the readiness with which they placed specimens 
from their collections at my disposal. 
My thanks are also due to the Executive Committee of the Carnegie Trust for a 
grant to defray the expenses of illustrating this paper. 
* “On the Fossil Osmundacee,” Trans. Roy. Soc. Hdin., 1908-1910. 
