from the Silun'an Rorks of North America. 179 



'1. Ptilodictya emacerata, Nich. PI. XIV. figs. 2— 2 b. 



Polyzoary consisting of minute, narrow, linear fronds, 

 ■uhich branch dichotomously, and have the form of a much 

 flattened, acutely pointed ellipse in transverse section. Width 

 one third of a line ; length of largest specimen observed two 

 lines. Cells elliptical, their long axes corresponding with 

 that of the branches, about six or seven in the space of one 

 line measured longitudinally. There are four, five, or rarely 

 six rows of cells in the frond. When four rows of cells are 

 present, two of these (in the centre) are longitudinal, and one 

 row on each side is composed of cells directed in an obliquely 

 ascending manner. When there are five rows, as is most 

 commonly the case, the three central ones are longitudinal and 

 a lateral row on each side is oblique. When there are six 

 rows, two central ones are longitudinal and two on each side 

 oblique. The cell-mouths are much longer than wide, and 

 each row is separated from the next by an elevated line. The 

 lateral margin of the frond on each side forms an obtuse non- 

 celluliferous edge, the width of which is so small that it cannot 

 always be detected. A central axis was not clearly determined, 

 but is doubtless present. 



The only previously recorded species of the genus to which 

 Ptilodicti/a emacerata presents any close resemblance is P. 

 fragilis, Billings, from strata of the same age in Anticosti 

 (Cat. Sil. Foss. of Anticosti, p. 9). Our species, however, is 

 distinguished from the latter by its uniformly more minute 

 dimensions, the smaller number of rows of cells in the frond, 

 and the possession in general of no more than a single row of 

 oblique cells on each side. P. fragi'lis, on the other hand, 

 has a width of from two thirds of a line to one line, with 

 from eight to ten rows of cells, and two or three rows of 

 oblique marginal cells on each side. It is possible our form 

 is only a variety of P. frag ill's ; but in the absence of figures of 

 the latter, and in the ffice of the differences above mentioned, 

 I think it safest to regard P. emacerata as a distinct species. 



Locality and Formation. — Cincinnati Group, near Cincin- 

 nati, Ohio. Collected by Mr. U. P. James. 



3. Ptilodictya jlagellum^ Nich. PI. XIV. figs. 3-3 Z». 



Polyzoary consisting of a single, narrow, unbranched, two- 

 edged, flattened frond, which has an acutely elliptical section. 

 The frond commences at an attenuated base, and gradually ex- 

 pands till a width of one line is reached, the total length of the 



