Whitfield and Dawson—Nature of Dictyophyton. 133 
in breadth. The longitudinal bands are ooo 3™™ broad, the 
transverse bands much narrower. Some of the rectangular in- 
terspaces are of the color of the ae athe wholly or par- 
tially stained with dark matter. e meshes are near! y black, 
but ina bright light show a fibrous texture and metallic lus- 
ter due to pyrite. 
Viewed as opaque objects under the microscope, the reticu- 
lating bands are seen to be fascicles of slender cylindrical rods 
or spicules, varying much in diameter; some of the largest 
being in the narrow transverse bands. The spicules may, in a 
few cases, be seen to be tapering very gently iis a point, but 
usually seem quite cylindrical and smooth. their present 
state they appear as solid phining a of ete The largest 
spicules are about 335 of an inch in diameter ; baa smaller 
n the whole the structures are not ideutiaal with those of 
any vlan ee to me, and rather resemble those of siliceous 
sponges of the genus Huplectella, 
e most puzzling fact in connection with this view is the 
mineral condition of the spicules now wholly replaced by 
pyrite. Carbonaceous structures are often replaced in this way 
s 
siliceous organisms. If the spicules were originally siliceous, 
either they must have had large internal cavities which have 
been filled with pyrite, or the original material must have been 
wholly dissolved out and its place occupied with pyrite. It is 
to be observed, however, that in fossil sponges the siliceous 
matter has not infrequently been dissolved out, and its space 
left vacant or filled with other matters. ave specimens of 
Actylospongia from the ‘Nant formation which have thus ager 
replaced by ae of a ferruginous color; and in a bundle of 
fibers probably a sponge allied to Hyalonema from the 
Piandailo of of Scotland, I find the substance of the spicules 
entirely gone and the spaces ‘formerly occupied by them empty. 
It should be added that joints of Crinoid stems and fronds of 
Fenestella occurring in the same specimen with the Uphantaenia 
are apparently in their natural calcareous state. 
Though I have hitherto regarded this curious organism as a 
fucoid, I confess that the study of the specimen above referred 
to inclines me to regard it a more preety a sponge. 
I owe the opportunity of examining this very interesting 
specimen to the kindness of Sheath Whitfield. 
