580 PALEONTOLOGY OF ILLINOIS. 



ARCHIMEDES LAXUS Hall. 



PL LXHI, flg. 15-15a. 

 Archimedes laxa Hall, 1857. Proc. Amer. Ass. Ad. Sci. Vol. 10, p. 178. 



Helicopora archimediformis Claypole, 1883. Quar. Jour. "Geol. 8oc. Vol. 39, p. 34, PI. 

 IV, flg. 3,4. 



Axis very loosely enrolled, consisting of scarcely more than 

 the thickened edge of the spiral frond; completing a volution 

 at intervals of 2.5 cm. or more. Number of volutions probably 

 not more than four. Fenestrated expansion diverging from the 

 axial line at an acute angle, wide, the greatest width observed 

 5.5 cm., slightly decumbent toward the free margin. On the 

 reverse the branches are narrowly rounded, becoming zigzag 

 and thinner as they near the outer margin; 20 or 21 in one 

 cm. Fenestrules oblong quadrate, or somewhat hexagonal, as 

 wide or wider than the branches, with a length equal to nearly 

 twice the width; about 13 in one cm. Dissepiments usually as 

 strong or stronger than the branches, and sometimes more 

 prominent. On the obverse the fenestrules are narrow, ellipti- 

 cal, the branches stronger and rigid, subangular, but without 

 a keel. Zooecia with rather small circular apertures more than 

 their diameter apart, and very slight peristome, in two ranges 

 excepting just below a bifurcation where some of the branches 

 exhibit three rows; 21 or 22 in five mm. 



As above cited, Prof. Claypole referred a specimen of this spe- 

 cies to his new genus Helicopora. While agreeing that A. laxus 

 closely approaches his genus, I am still indisposed to accept his 

 arrangement, since it scarcely admits of question that the form 

 is nothing more than a loosely coiled Archimedes. The Silurian 

 type of his genus (H. latispiralis) does not appear to be so. 



Position and locality: Chester group. Fragments are not un- 

 common at Chester, Illinois, and Litchfield, Ky. 



LYROPORA Hall, 1857. 



(Proc. Amer. Ass. Ad. Sci., vol. 10, p. 179.) 

 (For generic diagnosis see page 390.) 



This genus, although presenting but few characters to distin- 

 guish it from either Fenestella or Polypora, is nevertheless a 

 very convenient and natural group of Lower Carboniferous 



