172 



Cincinnati Society of Natural History. 



flabellate or undulated expansions, which are celluliferous on two 

 faces, and have no distinct non-poriferous margin. Surface smooth. 

 Cells arranged between rather prominently elevated longitudinal striae, 

 and in more or less regular transverse or diagonally intersecting series. 

 Measured longitudinally nine cells occupy the space of .1 inch; trans- 

 versely from nine to twelve occup} T the same space. The cell-aper- 

 tures are ovate to sub-quadrate, and the walls thick. Scattered over 

 the surface at variable intervals are small, apparently solid spaces, 

 which do not however interrupt the regularity of the arrangement of 

 the cells. 



Vertical sections show that the cells arise rather abruptly from the 

 axial laminae, near which their walls are veiy thin. Soon, however, the 

 walls are suddenly thickened, and appear to contain interstitial 

 cells, since each wall is crossed by two or three dark lines, which ap- 

 pear to be diaphragms. Between this zone and the outer surface, the 

 sclerenchyma between the walls of adjacent tubes is seemingly struc- 

 tureless. In a single section of this kind the axial laminae appear to 

 be perforated. No diaphragms have been observed in the tubes of the 

 proper zocecia. 



In tangential sections the transverse section of the tubes may 

 present four different aspects, according to the distance from the 

 median laminae at which they are cut. Just above the central axis the 

 cells are thin-walled and quadrate, and arranged between straight 

 longitudinal lines. The lines between the ends of the cells are alwa} T s 

 slightly curved, and usually constitute flexuous transverse lines, that 

 cross the longitudinal lines at either a right angle, or somewhat 

 obliquely. A little nearer to the surface the cells become elliptical and 

 have thin ring-like walls; the space between the ends of the cells is 

 occupied by calcite, which in the third stage (PI. VIII., fig. lb) is 

 filled by structureless sclerenchyma. In the fourth stage, representing 

 a transverse section of the tubes immediately below the surface, the 

 longitudinal lines are replaced by a close series of minute spiniform 

 tubuli, and the appearance presented at this stage is almost identical 

 with that of a like section of Bhinidictya nicholsoni (PI. VIII., fig. 66). 



Of the three species, Phoenopora explanata, P. constellate^ and P. 

 ensiformis, originally referred to Pheenopora by Hall (Pal. N. Y., vol. 

 ii., 1852), the last is an undoubted species of Ptilodictya (as restricted), 

 since it po ssesses the pointed articulating process and cellular struc- 

 ture of that genus. The second species I believe should also be re- 

 ferred to that genus, since none of its characters, so far as I have been 



