570 PALAEONTOLOGY OF ILLINOIS. 



slightly carinated on the obverse, and rounded on the reverse. 

 Zooecia about 22 in five mm., in two ranges. Fenestrules sub- 

 quadrate, longer than wide, 1(5 in one cm. Dissepiments as 

 strong as the branches. 



Diameter of shaft 6 mm.; diameter of flange 2 cm.; number of 

 volutions in 10 cm,, 8, 9 or 10. 



This fine species is related to both A. owenanus and A. wor- 

 theni. From the first the axis differs in having a wider flange 

 and longer volutions, from the second in having the under side 

 of the flange smooth, and in the outline of the shaft. 



Position and locality: Keokuk group. All the specimens seen 

 are from Jersey Co., 111. 



ARCHIMEDES OWENANUS Hall. 



PI. LXHI, Fig. 6-6c. 

 Archimedes oicenana Hall, 1857. Proc. Amer. Ass. Ad. Sci., vol. 10. p. 178. 



Axis probably very long, sometimes slender, at other times 

 very strong; volutions disproportionately rapid at the proximal 

 end, but soon they become longer; generally 2.5 occur in two 

 cm., more rarely three, and occasionally only two. The shaft 

 varies considerably in strength in different examples. Just 

 above the flattened top of the flange it is sub-cylindrical, spread- 

 ing a little in young examples toward the base of the succeed- 

 ing flange, which bends abruptly outward, the junction between 

 the two being marked by a more or less defined sulcus. Fenes- 

 trated expansion delicate, several cm. wide, leaving the axial 

 line at an angle of about 70. Reverse only seen. Branches 

 narrowly rounded, 22 in 1 cm. Fenestrules oblong quadrate or 

 elliptical, as wide as the branches, the length equal to one and 

 a half times the width, with 13 or 14 in 1 cm. Dissepiments as 

 strong as the branches. 



Diam.of shaft of strongestex., 5.7mm.; No. of vol. in 2 cm., 2.3. 



" smallest " 1.8mm.; " " 2.7. 



" " " average " 2.9mm.; " " " 2.2. 



The above description refers to the form usually identified 

 with Hall's A. owenanus. The very incomplete original descrip- 

 tion contains nothing that does not agree with it. The posi- 

 tion of the species is somewhat intermediate between A. wor- 



