CORALS AND BRYOZOA. 
39 
them. Rows of apertures separated by prominent granulose ridges. Between 
each two adjacent apertures are two minute triangular pits. 
This species differs from P. tenuis (pi. xvii, figs. 7—13) in its smaller size, 
proportionally longer cells and the lirate appearance given to it by the ridges 
separating the longitudinal lines of cell apertures. 
Formation and localitij. In the upper shaly portions of the Lower Helderberg 
group, near Clarksville, N. Y. 
Ptilodictya tenuis. 
PLATE XVII, FIGS. 7-12 ; PLATE XIII, FIG. 14; AND PLATE XXIII A, FIG. 15. 
Escharopora tenuis, Hall. Twenty-sixth Rept. N. Y. State Mus. Nat. Hist., p. 99. 1874. 
“ “ “ Thirty-second Rept. N. Y. State Mus. Nat. Hist., p. 161. 1879. 
In part Escharopora tenuis. Hall. Rejiort of State Geologist for 1882. Expl. pi. 17, figs. 7-13. 1883. 
Zoarium consisting of a thin elongate stipe, with a thickened base; gradually 
enlarging from an obtuse point, for a short distance, beyond which the sides 
become essentially parallel; width varying from 3 to 7 mm. ; transverse 
section lenticular; greatest thickness 1 mm. Cells tubular, arising obliquely 
from the mesotheca. Apertures oval or subquadrangular, though frequently 
assuming other forms from thickening by silicification ; disposed in longitudi¬ 
nal parallel ranges; on the central portion of the stipe they are essentially 
of the same size; length .30 nun., width .20 mm.; marginal cells variable in 
size and arrangement and diverging from the axis of the frond. Between 
adjacent cell apertures there are sometimes two minute cellules. Cell walls 
of the parallel ranges in contact laterally, generally coalescing, and very 
often becoming much thickened by silicification, especially on the surface, 
where they form strong longitudinal ridges, separating the rows of cell 
apertures ; outer portion of the cell wall very thin. 
This species is associated with P. lirata, and further investigation may show 
that the two forms represent different conditions or stages of growth of a single 
species. 
Formation and localities. In the shaly limestone of the Lower Helderberg 
group, Albany and Schoharie counties, N. Y. 
