Hasten . — The Structure of Mesoxy Ion Sutcliffii (Scott). 41 1 
centripetal wood of the leaf-traces which is present in the outer part of 
their course is entirely lost before they reach the boundary of the pith. As 
Dr. Scott says: ‘In the Cordaiteae the old “ Cryptogamic ” or centripetal 
wood appears to have been on the verge of extinction, and its presence or 
absence may here be of little taxonomic significance.’ 1 This being the 
case, Mesoxylon , which resembles Cordaites in so many of its other characters, 
may best be included in the family Cordaiteae. The affinity of the French 
Poroxyleae with the Cordaiteae has been recognized ever since the descrip- 
tion of the forms by MM. Bertrand and Renault. The discovery of the 
leaves of Poroxylon , and the attribution to this form of the platyspermic 
seed known as Rhabdocarpus , Br., by Grand’Eury, if the discovery is con- 
firmed, considerably strengthen the affinity otherwise indicated between 
the Poroxyleae and Cordaiteae. Marks of similarity between the Poroxyleae 
and Lyginodendron and Heterangium were also indicated by the P'rench 
observers, and at the present time it is well recognized that the agreement 
in the structure of the stem between Poroxylon and Lyginodendron is in 
many respects a close one, although, with a common type of stem structure, 
there existed in the one form a foliage type which in form and structure 
resembled that of Cordaites and recent megaphyllous Gymnosperms, while 
in the other the foliage was in every respect that of a fern. With Calamo- 
pitys Saturni , Unger, again, Poroxylon has much in common. 
In consequence of the obvious affinities of Poroxylon in both directions, 
and of the very perfect manner in which the tissues have been preserved, 
and the very detailed account which we owe to MM. Bertrand and Renault, 
the Poroxylons have come to possess a crucial significance in the discussion 
of the relation of the Cordaiteae to the Pteridosperms, and so on that of the 
broader question of the derivation of the higher Gymnosperms. 
The species of Poroxylon were described from deposits of Permo- 
Carboniferous Age, and thus can hardly be considered as forming an actual 
link between families which had been fully differentiated long before that 
time ; it is therefore particularly interesting to find, at a much lower 
horizon — the Lower Coal Measures of Lancashire — forms which, while 
possessing centripetal wood as in Poroxylon , are in other respects much 
closer to Cordaites , and probably stand nearer to the direct line of Cor- 
daitean descent. Another Coal-Measure plant, Dadoxylon Spenceri , Scott, 
already briefly described (p. 395), although differing from Mesoxylon Sutcliffii 
and Cordaites in the small size of the pith, resembles Mesoxylon in so 
many other respects that it may prove to be closely related. Pitys antiqua , 
again, a Lower Carboniferous form (see p. 395), agrees with Mesoxylon in the 
structure of its wood excepting for the greater width of the medullary rays 
and the definitely mesarch structure of its single xylem strands surrounding 
the pith ; judging from the anatomical structure of the pith and wood (the 
1 Studies, and Edition, p. 526. 
