628 PALEONTOLOGY OF ILLINOIS. 



SEPTOPOKA CESTKIENSIS Prout. 



PI. LXIV, flg. 1-16. 



Septopora cestriensis Prout. 1859. Trans. St. Louis, Acad. Sci. Vol. I. p. 448, PI. 18, 



figs. 2-2& (pars). 

 (Not Septopora cestriensis Meek & Worthen, 1870. Geol. Surv. 111. vol. V, PI. 24, flg. 



14a-14c.) 



Zoarium a rather delicate but compactly woven flabellate ex- 

 pansion. Branches not very rigid, averaging from twelve to 

 fourteen in 1 cm., increasing in number by bifurcation and not 

 by interpolation, with an average width of 0.5 mm., but en- 

 larging from 0.3 or 0.4 mm. immediately after bifurcation to 

 0.7 or 0.8 mm. before the next division. 



Reverse. Branches narrowly rounded, slightly zigzag or 

 straight, striated w r hen young and smooth on mature or old 

 examples; accessory pores rather large, about 0.09 mm. in di- 

 ameter, not very numerous, situated generally opposite the 

 connecting branchlets or dissepiments. The latter are generally 

 a little thinner than the branches and on the same level with 

 them. Fenestrules varying in shape from circular or oval to 

 sub-quadrate, commonly between ten and eleven in 1 cm.; but 

 some otherwise typical specimens have twelve or thirteen in 

 that space. 



Obverse. Branches sub-angular, with a faint mesial keel, 

 which is most prominent on worn examples; generally the keel 

 expands at intervals of 1 mm. or less into rather prominent 

 elongated nodes. The dissepiments extend directly across from 

 branch to branch, and expand at their junction with them, 

 causing rounded fenestrules to prevail. Near the outer margin 

 of the frond the fenestrules are likely to be sub-quadrate. Zooe- 

 cia in two ranges, but -a third is occasionally interpolated just 

 before a bifurcation. Apertures circular, 0.14 mm. in diam- 

 eter, about two-thirds of their diameter apart, twenty-two or 

 twenty-three in 5 mm. Peristome thin and but little elevated. 

 On the dissepiments the rule is to find four apertures, two on 

 each side. Often, however, a fifth will occupy the central point, 

 while at other times the number is reduced to three or even 

 two. A small number of the accessory pores occur among the 

 zooecia apertures, but on this side they are usually restricted 

 to the dissepiments. 



