158 THE PALEONTOLOGY OF MINNESOTA. 
: |Pachydictya. 
In the very young specimens, or at the distal extremities of the branches of the 
more mature ones, the zoccial apertures are comparatively large, the lateral inter- 
spaces correspondingly narrow, and the end spaces with one or two depressions. In 
this stage the interspace granulations are very faint, but in the succeeding stages they 
are much better defined, the apertures often smaller, with the width of the inter- 
spaces increasing with greater rapidity, the increase in the circumference of the 
branches being divided between the lateral interspaces. In most of these specimens 
the interspaces are now flat or faintly concave, with a more or less distinctly recog- 
nizable though thin peristome about the apertures. In others a row of the inter- 
stitial papille occupies a faint longitudinal ridge, that may be elevated to slightly 
above the level of-the peristomes. In more rare instances the peristomes appear to 
be wanting over parts of the surface, and the whole interspace convex and irregularly 
granulose, and seeming to slope down into the apertures. These specimens have 
quite a different aspect from the ordinary form of the species, indeed, so much so, that 
I mistook them for a species of Rhinidictya. Non-poriferous margin never wide, 
often so narrow as to be practically wanting. Its surface is papillose. Not infre- 
quently large patches of the surface, where the zocecial apertures have been closed 
by a thin deposit of calcareous material, are covered with such papille. 
Internal characters vary much as in P. acuta, excepting that they are all a little 
smaller, and the transverse walls between the prostrate cells of the zoccial tubes 
straighter. 
When the preliminary description of this species was written I had unfortunately 
mislaid the two specimens regarded as the types of the form named Ihinidictya 
humilis, and which I believed to have been derived from the lowest shales at Minne- 
apolis. In preparing the Minnesota material for my final studies they were found 
and the label with them proves that they were really collected at the same time and 
from the same beds as the original specimens of P. pumila. Later washings of the 
shales from this locality have added greatly to the number of specimens. With 
this more complete representation of the species I have become satisfied that the 
supposed Rhinidictya exhibits merely another phase of surface marking of P. pumila, 
deserving not even subordinate distinction. Among the lot, however, there is a form 
of which I have over twenty specimens, that might be distinguished as var. swblata. 
The zoarium does not appear to have been much larger than in the typical form, but 
its branches are wider, and though there are generally two or three rows of zocecia 
more than in the largest of the type form, the greater width of the branches ‘is 
chiefly due to a wide non-poriferous margin. 
