44 GEOLOGICAL AND NATURAL HISTORY SURVEY OP CANADA. 



Pachydictya acuta, Hall. 



Stictopora ? acuta, Hall, 1847, Pal. N.Y., vol. 1, p. 74, pi. 26, fig. 3a-36. 

 Stictopora acuta, Ulrich, 1882, Jour. Cin. Soc. Nat. Hist., vol. 5, p. 168, pL 8, fig, 

 1-lb. 



Pachydictya acuta, Ulrich, 1886, ]4th Ann. Kept. Minn. St. Geol, p. 67. 



A fragment of this widely distributed species was collected by Di-. 

 R. Bell, in 1880, from a cream coloured or buff limestone of the 

 Trenton group at St, Andrews, Manitoba. 



Ptilodictya Whiteavesi. (X. Sp.) 

 P]. VIII., figs. \-\c. 



Zoarium consisting of a single unbranched, two-edged bifoliate frond^ 

 which is either a little curved or sti aight, and expands upward from the 

 finely striated articulating " head." The central region of the frond is 

 raised into a low broad ridge, and upon this the apej-tures of the zooecia 

 are I'anged into seven or more modei'ately regular longitudinal series 

 between more or less distinctly elevated longtitudinal lines. Young 

 examples, like that represented by fig. le, have, with the exception of 

 an oblique series along the sharp non-celluliferous border, only longi- 

 tudinally arranged zooecia. Fig. Id represents an older specimen in 

 which the frond has be^un to widen, and fig. 1 is a restored represent- 

 ation of a fully matured example. At this stage of growth the surfaces 

 on each side of the median ridge are slightlj^ concave or flat, and upon 

 them the apertures of the zooecia are much less regularly arranged, 

 these latter being also mostly of hexagonal shape. Somewhat irregu" 

 larly distributed clusteis of large cells constitute a marked superficial 

 feature of these lateral areas. The largest cells in these clusters are 

 0'35 mm. in diameter, while the aveiage diameter of the cells occupy- 

 ing the intermediate spaces is about 0*2 mm., there being about twelve 

 of the latter in three mm. The zooecia on the median ridges have 

 oval apertuies situated at the bottom of sloping areas of subquadrate 

 outline; there between nine and ten occur in three mm. longitudinally"^ 

 and nearly five of the rows in one millimetre transversel3^ 



The internal characters, as brought out by tangential and vertical 

 sections, are very well represented in figs. \b and le. The vertical 

 section shows that the superior hemiseptum is well developed and 

 recurs at successive stages in the growth of the zoarium. A complete 

 diaphragm also occasionally intersects the tubes near their mouths. 



