POLYZOA FROM THE SILURIAN FORMATION. 



MOLLUSCA. 

 POLYZOA. 



Genus PTILODICTYA, Londsdale, 1839. 

 ' (Murchison's Sil. Syst., p. 130.) 



Ptilodictya falciformis, Nicholson. 



Plate 25, figs. 7, 76. 



Polyzoary consisting of a single unbranched, or slightly branched, 

 elongated, flattened, narrow, and two-edged frond, the form of which is 

 more or less curved and 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 rarely or never 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 are not angulatefl. No central axis can, as a rule, be made 

 out with certainty, though the existence of such can sometimes be 

 demonstrated. The edges of the frond are thin and sharp, formed by a 

 narrow band, which is longitudinally striated, and, when perfect, is per- 

 forated by the apertures of minute imperfect cells, which have a longi- 

 tudinal direction. Both sides bf the frond are celluliferous, the cells 

 being apparently perpendicular to the surface, and being arranged in in- 

 tersecting diagonal lines, which form angles of about thirty degrees with 

 the sides of the frond, and thus cut one another at sixty degrees. The 

 mouths of the cells are oval, or somewhat diamond-shaped, their long 

 axis coinciding with the axis of the frond, alternately placed in contigu- 

 ous rows, about eight in the space of one line measured diagonally, and 

 ten in the same space measured transversely, the outermost rows very 

 slightly smaller than the others. Walls of the cells moderately thick ; 

 no surface-granulations, tubercles, spines, or elevated lines. The mouths 



