592 PALAEONTOLOGY OF ILLINOIS. 



mediately after bifurcation, these soon increase to three, and 

 then to four before the next division. Apertures circular, rayed, 

 with prominent peristome, less than twice their diameter apart, 

 about twelve in 3 mm. On the reverse the branches are rather 

 straight, the dissepiments half as wide, the fenestrules sub- 

 quadrate or elliptical, and twice as long as wide. 



This beautiful little species is distinguished from its nearest 

 relative P. retrorsa, by its closer branches, smaller fenestrules, 

 their more elongate form, smooth and straighter branches on 

 the reverse, and the direct apertures. The rayed apertures are 

 also better shown in P. radialis than in any other species of 

 the genus known to me. 



Position and locality: Keokuk group. Rare at Keokuk, Iowa. 



POLYPOKA BISERIATA UMch. 



PI. LX, figs. 4-46. 



Zoarium a slowly expanding, irregular, more or less undulat- 

 ing, foliar net-work, from four to eight cms. in height. Branches 

 closely approximated, seventeen to nineteen in 1 cm., slender, 

 averaging 0.5 mm. in width, but increasing from 0.4 to < .7 

 mm. before bifurcation, which takes place at intervals of from 

 5 to 14 mm. Their poriferous surface is nearly flat or slightly 

 elevated centrally, where a row of prominent spines or nodes, 

 about 0.45 mm. apart, almost gives the impression of a median 

 keel. Dissepiments short, depressed, one-half or less than one- 

 half the width of the branches. Fenestrules small, sub-oval, 

 about fourteen in 1 cm. Zooecia in two alternating ranges, in- 

 creasing to three at a point 2 or 3 mm. below the branch di- 

 visions. Apertures large, 0.15 mm. in diameter, direct, with 

 prominent peristome, seventeen or eighteen in 5 mm., their di- 

 ameter or less apart, often closed by an opercular cover of the 

 usual type. On the reverse the branches are rather broadly 

 rounded, somewhat channeled below the bifurcations, and either 

 smooth or very finely striated; the dissepiments thin and not 

 depressed, and the fenestrules sub-quadrate. 



The material upon which this species is founded is very satis- 

 factory, many of the examples being free and in a fine state of 

 preservation. The species is very constant and all the speci- 



