246 DuPpLEeR: AIR CHAMBERS OF REBOULIA 
radiate pinnately toward the margin. The secondary partitions 
are for the most part parallel with or slightly oblique to the primary 
walls, and give a very areolated structure to the thallus, only the 
number and distribution of the air pores giving any indication as 
to the primary chambers. A cross section of the thallus in the 
midrib region shows several layers of superimposed chambers in 
this region (Fic. 1), the chambers being gradually reduced to a 
single layer along the margin (Fic. 2). The compact ventral 
tissue also gradually becomes narrower until at the margin of the 
thallus it is usually only a single layer of cells in thickness. Longi- 
tudinal sections (F1G. 3) show that this appearance of superimposed 
chambers is due largely, if indeed not altogether, to an extensive 
shingle-like overlapping of the primary chambers, greatly com- 
plicated by the formation of partial secondary chambers by parti- 
tions which form in the chambers. These primary chambers 
extend from the surface to the compact ventral tissue. Of course, 
“deeper” chambers appear in sections cut in almost any direction, 
but a careful study of these chambers through a complete series 
of sections shows them to be largely the more deeply situated 
portions of the primary chambers. Where the tissue is more 
compact, as in Plagiochasma and Grimaldia fragrans, the deep 
secondary chambers may be more probable. An examination of 
a few preparations of Neesiella rupestris indicates that the inter- 
pretation here applied to Reboulia also applies to that form, which 
has a-more simply organized tissue than Reboulia. Cross sections 
give an appearance similar to that described for Grimaldia by 
Evans (7), the dorsal chambers being partially subdivided by 
plate-like outgrowths arising from the sides and floor of the 
chambers. Sections parallel to the surface are helpful in the 
interpretation of the structure (Fic. 5). It is very doubtful 
whether any plates arise from the roof of the chamber and project 
downwards into the chambers, the appearance of this in sections 
being due to the oblique inclination of the plates. Many of these 
secondary plates come in contact with the roof of the chambers. 
The primary partitions, the secondary plates and the roof of the 
chamber are all but a single layer of cells in thickness. Occa- 
sionally the cone formed by the air-pore projects inward into the 
chamber, this condition being associated with the portion of the 
thallus immediately posterior to the male receptacle. 
