On new Species of Silurian Polyzoa. 177 



XXII. — Descriptions of new Species of Polyzoa from the Lower 

 and Upper Silurian Rocks of North America. By II. 

 ALLiiVNii Nicholson, M.D., D.Sc, F.R.S.E., Professor of 

 Biology in the Durham University College of Piiysical 

 Science, Newcastle-on-Tyne. 



[Plate XIV.] 



Having in a former communication described the species of 

 Alecto and Hijypotlioa which have come under my notice as 

 occurring in tlie 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 J Nich. Pi. XIV. figs. \-\b. 



Polyzoary consisting of a single, unbranclied, 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 Avidth, 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 mai-ked with longitudinal or 

 slightly oblique strite 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 lines. The mouths of the cells parallel 

 with the general surface, neither lip being especially prominent, 

 and the plane of the aperture not being oblique. 



