i6o 



NORTH AMERICAN INDEX FOSSILS. 



and the greater interspaces between the apertures, which are much 

 larger near the margin than at the center. 



Trenton of New York, Kentucky, Iowa, Minnesota and Manitoba. 



142. P. crassa (Hall). Siluric. 



Branches 2.5 to 3 mm. wide ; cells elliptical, with strong peris- 

 tomes and separated by slightly wavy ridges; a marginal row of 

 larger apertures, with a non-celluliferous striated margin. 



Clinton of New York, Ohio and Ontario ; Niagara of New 

 York ; Anticosti group of Anticosti. 



LXXXIII. Cystodictya Ulrich. 



Ramose, of two layers of cells grown back to back, cross sections 

 elliptical ; subparallel margins non-poriferous ; apertures subellip- 

 tical in linear series between longitudinal ridges, with lunarium on 

 marginal side of apertures ; interspaces finely striated, granulose 

 or smooth. Worn specimens show pits and cells. Dev.-Carb. 



143. C. gilberti (Meek). (Fig. 214, b.) Devonic. 

 Repeatedly branching rows of apertures and separating ridges, 



increasing rapidly by interpolation on the branches. 



Onondaga to Hamilton 

 of Ohio and Ontario ; Ham- 

 ilton of Falls of the Ohio 

 and Utica, Indiana. 



144. C. hamiltonensis Ul- 

 rich. Devonic. 

 Branches bifurcating, 2.5 



to 3 mm. wide ; apertures 

 nearly circular, alternating 

 in adjoining rows. 



Hamilton of Iowa, Illi- 

 nois, Wisconsin, Western 

 New York and Manitoba. 



145. C. incisurata (Hall). (Fig. 211.) Devonic. 

 Branches bifurcating from 2 to 6 mm. in width, margins parallel 



or slightly diverging; ridges continuous to the margin, where they 

 often cause denticulations, slowly increasing by intercalation ; 

 apertures circular to oval or irregular ; lunarium strong. 

 Hamilton of Central and Western New York and Ontario. 





Fig. 211. Cystodictya incisurata. a, natural 

 size, and b, portion enlarged. (After Hall and 

 Simpson.) 



