from the Silurian Rocks of North America. 179 
2. Ptilodictya emacerata, Nich. Pl. XIV. figs. 2-2 6. 
Polyzoary consisting of minute, narrow, linear fronds, 
which branch dichotomously, and have the form of a much 
flattened, acutely pointed ellipse in transverse section. Width 
one third of a line; length of largest specimen observed two 
lines. Cells elliptical, their long axes corresponding with 
that of the branches, about six or seven in the space of one 
line measured longitudinally. There are four, five, or rarely 
six rows of cells in the frond. When four rows of cells are 
present, two of these (in the centre) are longitudinal, and one 
row on each side is composed of cells directed in an obliquely 
ascending manner. When there are five rows, as is most 
commonly the case, the three central ones are longitudinal and 
a lateral row on each side is oblique. When there are six 
rows, two central ones are longitudinal and two on each side 
oblique. The cell-mouths are much longer than wide, and 
each row is separated from the next by an elevated line. The 
lateral margin of the frond on each side forms an obtuse non- 
celluliferous edge, the width of which is so small that it cannot 
always be detected. A central axis was not clearly determined, 
but is doubtless present. 
The only previously recorded species of the genus to which 
Ptilodictya emacerata presents any close resemblance is P. 
fragilis, Billings, from strata of the same age in Anticosti 
(Cat. Sil. Foss. of Anticosti, p. 9). Our species, however, is 
distinguished from the latter by its uniformly more minute 
dimensions, the smaller number of rows of cells in the frond, 
and the possession in general of no more than a single row of 
oblique cells on each side. P. fragilis, on the other hand, 
has a width of from two thirds of a line to one line, with 
from eight to ten rows of cells, and two or three rows of 
oblique marginal cells on each side. It is possible our form 
is only a variety of P. fragilis ; but in the absence of figures of 
the latter, and in the face of the differences above mentioned, 
I think it safest to regard P. emacerata as a distinct species. 
Locality and Formation.—Cincinnati Group, near Cincin- 
nati, Ohio. Collected by Mr. U. P. James. 
3. Ptilodictya flagellum, Nich. Pl. XIV. figs. 3-3 6. 
Polyzoary consisting of a single, narrow, unbranched, two- 
edged, flattened frond, which has an acutely elliptical section. 
The frond commences at an attenuated base, and gradually ex- 
pands till a width of one line is reached, the total length of the 
