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 Urate appearance given to it by the ridges 

 separating the longitudinal lines of cell apertures. 



Formation and locality. In the upper shaly portions of the Lower Helderberg 

 group, near Clarksville, N. Y. 



Ptilodictya tenuis. 



PLATE XVII, FIGS. 7-l.i ; PLATE XIU, FIG. U; AND PLATE XXIU A, FIG. 16. 



Bscharopora tenuis, Hall. Twenty-sixth Rept. N. Y. State Mus. Nat. Hist., p. 99. 1874. 



" " " Thirty-second Kept. N. Y. State Mus. Nat. Hist., p. 161. 1879. 



In part Bscharopora tenuis. Hall. Report of State Geologist for 1882. Expl. pi. 17, fig-s. 7-13. 1S83. 



ZoARiDM 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 mm., 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. 



