On new Species of Silurian Polyzoa. 177 
“XXI1I.—Descriptions of new Species of Polyzoa from the Lower 
and Upper Silurian Locks of North America. By H. 
ALLEYNE Nicuoxson, M.D., D.Sc., F.R.S.E., Professor of 
Biology in the Durham University College of Physical 
Science, Newcastle-on-Tyne. 
[Plate XIV.] 
HAVING in a former communication described the species of 
Alecto and Hippothoa which have come under my notice as 
occurring in the Cincinnati Group (Lower Silurian) of Ohio, 
I have now to describe from the same formation several species 
of Ptilodictya and one of Ceramopora, which I have been able 
to determine, from the collections submitted to me by Mr. U. 
P. James and Prof. Edward Orton, and all of which appear 
to be new. I have also an interesting species of Fenestella 
to describe, from the Upper Silurian (Guelph division of the 
Niagara formation) of the State of Ohio. 
1. Ptilodictya falciformis, Nich. Pl. XIV. figs. 1-1 6. 
Polyzoary consisting of a single, unbranched, or slightly 
branched, elongated, flattened and two-edged frond, the form 
of which is curved or falciform, and which gradually expands 
from a pointed base till it reaches a width of two lines within 
a distance of less than half an inch above the base. The total 
length may exceed two inches; but the width, in typical ex- 
amples, rarely exceeds two and a half lines. ‘The transverse 
section is acutely elliptical, the thickness in the middle not 
exceeding half a line; and the flat faces of the frond are very 
gently curved and not angulated. A central laminar axis, 
though often undemonstrable, can sometimes be clearly shown 
to exist. The edges of the frond are thin and sharp, formed 
by a narrow band, which is marked with longitudinal or 
slightly oblique strize and by the apertures of minute imperfect 
cells. Both sides of the frond are celluliferous, the cells 
being apparently perpendicular to the surface, and being 
arranged in intersecting diagonal lines, which form angles 
of about 30° with the sides of the frond, and thus cut one 
another at about 60°. The mouths of the cells are oval or 
somewhat diamond-shaped, their long axis coinciding with 
that of the frond, alternately placed in contiguous rows, about 
eight in the space of one line measured diagonally ; the outer- 
most rows very slightly smaller than the others. Walls of 
the cells moderately thick ; no surface-granulations, tubercles, 
spines, or elevated Jines. The mouths of the cells parallel 
with the general surface, neither lip being especially prominent, 
and the plane of the aperture not being oblique. 
