180 Dr. H. A. Nicholson on new Species of Polyzoa 
only specimen examined being eight lines. The general form 
of the frond is falciform, but towards the base it is alternately 
bent from side to side in a flexuous manner. The cells are 
arranged in longitudinal rows, about ten rows in the space of 
one line, the cells of contiguous rows alternating with one 
another. The cell-mouths, where most perfect, are narrow 
and long-oval—where worn, subcireular ; and the rows of cells 
are separated by strongly elevated longitudinal ridges. The 
non-celluliferous margins of the frond are inconspicuous; and 
the central axis, though doubtless present, was not clearly 
determined. 
This species most nearly resembles Pétlodictya gladiola, 
Billings, from which it is distinguished by its much smaller 
size and less width, and by its flexuous form. From P. falc?- 
formis, Nich., itis separated not only by the above characters, 
but also by the longitudinal arrangement of the cells. 
Locality and Formation.—Cincinnati Group, Lebanon, 
Ohio, immediately below the horizon of Streptelasma corni- 
culum. Collected by Prof. Edward Orton and Mr. W. Bean. 
4. Ptilodictya (2?) arctipora, Nich. Pl. XIV. figs. 4-40. 
Polyzoary forming a cylindrical, slightly branched frond, 
which is not sharp-edged, exhibits no non-celluliferous borders, 
and shows no traces of a central laminar axis. Cells arranged 
in obscurely longitudinal alternating rows, apparently perpen- 
dicular to the surface, and radiating in all directions from an 
imaginary axis. Cell-mouths very much compressed, much 
longer than wide, expanded below and attenuated superiorly, 
where they are often somewhat twisted and bent. Upon the 
whole, the cells are pyriform in shape, with their narrow ends 
directed upwards, about eight occupying the space of one line 
measured vertically, and twelve the same space measured 
diagonally. The cells are not always in contact, especially 
in their upper portion ; and their borders are always distinctly 
marked off by impressed lines; but they are not arranged 
between elevated longitudinal ridges. ‘The margins of the 
cells are very thick and conspicuous, not granulated, tubercu- 
lated, or spinigerous. 
The best-preserved fragment examined hada length of eight 
and a half lines, dividing at its summit into two branches, its 
diameter being rather more than one third of a line. 
From its cylindrical form, and the absence of a laminar axis 
or of non-poriferous margins, it would seem certain that this 
singular form is not a Ptilodictya ; but I am at a loss to know 
where it should properly be placed, its extreme minuteness 
