HOLDEN. — CRETACEOUS PITYOXYLA. 615 



On the other hand, Penhallow (14) has suggested that they have 

 been formed from ray parenchyma by a thickening of the cell wall. 

 As Bailey points out, the evolutionary sequence has been from thick- 

 to thin-walled parenchyma, not vice versa, — a consideration which 

 immediately invalidates this hypothesis. 



Pityoxyhn foliosum n. sp. 



The next specimen to be considered is much less like modern forms 

 than that just described. Figures a and b, Plate 3, represent at 

 different magnifications cross sections of the wood. As may be seen in 

 Figure b, the annual rings are very broad and well marked, — the first 

 occurring near the lower limit of the field. Another appears a little 

 below the centre of Figure a. There are many concentric arcs extend- 

 ing half way or more around the stem, caused by some external 

 pressure in the process of fossilization. One such is shown in the 

 upper part of Figure a. That it is not a true growth ring is proven 

 by the fact that it does not completely encircle the medulla, and that 

 the tracheides composing it are not pitted on the tangential wall, — 

 an invariable characteristic of the summer tracheides of the lignite 

 in question. 



Resin ducts are very numerous, extending in two planes, — vertical 

 and horizontal. The vertical canals are surrounded by clusters of 

 highly resinous epithelial parenchyma. Not infrequently a single 

 mass contains three or even four tangentially grouped canals, which 

 as may be seen from longitudinal sections, intercommunicate. The 

 epithelium is moderately thick-walled, and densely perforated by 

 simple pits. The horizontal canals are also numerous, as shown by 

 Figure e. Together with the vertical canals thc}^ form a freely anasto- 

 mosing system of resin passages throughout the wood. Both hori- 

 zontal and vertical canals, especially the latter, are almost invariably 

 filled with thick-walled tyloses. At the extreme right of Figure b the 

 proximity of the canals to the pith may with difficulty be ascertained. 

 Figure d, a radial longitudinal section, shows the relation more clearly. 

 In fact the canals are often so near to the medulla that in transverse 

 section they appear to be embedded in it. A more careful examination 

 however, reveals the presence of a jacket of metaxylem elements 

 around the duct. This occurrence of canals in the primary wood is 

 unknown in the main axis of living pines, but is similar to that of 

 Pinus protoscleropitys. 



Toward the right of Figure b, a vascular strand may be seen to pass 



