538 PALAEONTOLOGY OF ILLINOIS. 



Position and locality: Keokuk limestone, Keokuk and Ben- 

 tonsport, Iowa, Warsaw, Nauvoo, and other localities in Illi- 

 nois. Also in the Warsaw beds, at Warsaw. 



FENESTELLA LIMITARIS Ulrich. 



PL XLIX, flg. 4, 4a. 



Zoarium a convoluted foliar expansion, several cms. in width, 

 consisting of rather strong, rounded, subequal, and flexuous 

 rather than rigid branches; sixteen or seventeen in 1 cm. On 

 the obverse side the branches vary in width from 0.25 mm. just 

 above, to 0.5 just below a bifurcation, but the normal width is 

 fairly constant at 0.35 to 0.4 mm. Median keel obsolete, or 

 represented by a faint, somewhat flexuous, thread-like line, 

 scarcely rising above the cell-aperture margins. Zocecia in two 

 alternating rows, seven to two fenestrules, fifteen in 5 mm., 

 with rather large and more than usually direct, circular aper- 

 tures, surrounded by a prominent peristome. Dissepiments de- 

 pressed, narrowly rounded, thin, rarely half as thick as the 

 branches. Fenestrules narrow, elongate sub-elliptical, indented 

 by the projecting cell mouths, with an average of ten in 1 cm. 

 On the reverse they appear wider and more regular, and may 

 be said to be generally oblong quadrate in outline, with the 

 length equal to about twice the width. Here also the branches 

 are rather narrowly rounded, and more or less distinctly 

 striated. 



The very little developed median keel distinguishes this spe- 

 cies from F. rudis, its nearest congener. At the same time it 

 reminds us of species of Polypoid like P. biseriata, but as the 

 branches never carry more than two rows of cells it is evidently 

 still within the limits of Fenestella. 



Position and localit}^ Keokuk group. Rare at Bentonsport 

 and Keokuk, Iowa. 



FENESTELLA REGALIS Ulrich. 



PI. L, figs. 1, la, and PL LIV. flg. 5. 



Zoarium flabellate, usually forming a flat, sub-circular expan- 

 sion, 4 or 5 cms. in diameter. Branches very strong, rather 

 flexuous, bifurcating in the lower and middle portions of the 



