from the Silurian Rocks of North America. 181 
rendering its generic affinities very uncertain, owing to the 
impossibility of making out the details of its internal structure. 
It has, however, some affinity with Ptilodictya (?) raripora, 
Hall, from the Clinton Group; and I have therefore referred 
it provisionally to this genus. 
Locality and Formation.—Cincinnati Group, near Cincin- 
nati, Ohio. Collected by Mr. U. P. James. 
5. Ptilodictya fenestelliformis, Nich. Pl. XIV. figs. 5-5 d. 
Polyzoary palmate or subpalmate towards the base, dividing 
distally into small branches. Basal expansion and branches 
flattened and sharp-edged, the branches being acutely elliptical 
in cross section, and about three fourths of a line in thickness 
centrally. Cells covering the whole surface on both sides, 
with the exception of the sharp lateral margins, which are 
non-celluliferous, as well as of certain non-poriferous areas to 
be subsequently noticed. ‘The cells on the two aspects of the 
flattened frond respectively have their bases separated by a 
thin laminar axis. The cells in the middle of the frond are 
about three eighths of a line in height, gradually diminishing 
towards the margins. Cell-mouths ovate, slightly longer than 
broad, arranged in longitudinal rows, alternate or subalternate 
in contiguous rows; about five cells in one line measured 
longitudinally, and six in the same space measured diagonally. 
The longitudinal spaces between the rows of cells are broad 
and slightly elevated, and are faintly striated longitudinally 
or obscurely punctate. On the other hand, the spaces between 
the ends of the cells are very much narrower; and the surface 
thus closely resembles that of a small Fenestella—the cell- 
mouths looking like “fenestrules,” the longitudinal interspac s 
between the cells representing the “interstices,” and the narrow 
spaces between the ends of the cells corresponding with the 
“ dissepiments.” The only specimens examined exhibit nu- 
merous, apparently solid, rounded or stellate areole, of an 
average diameter of two thirds of a line, which are not occu- 
pied by cells, but which exhibit an obscurely pitted surface. 
In its superficial characters this form might readily be taken 
for a Fenestella, whilst the character last mentioned gives it 
somewhat the aspect of certain species of Chetetes (Monticu- 
lipora). Its internal structure, however, proves it beyond all 
question to be a genuine Ptilodictya ; and I am not acquainted 
with any other species of this genus with which it could be 
confounded. 
Locality and Formation.—Cincinnati Group, near Cincin- 
nati, Ohio. Collected by Mr. U. P. James. i 
Ann. & Mag. N. Hist. Ser.4. Vol. xv. 13 
