576 PALEONTOLOGY OF ILLINOIS. 



each extremity; volutions regular, varying in different exam- 

 ples from 3.7 to 5 in two cm.; shaft comparatively slender just 

 above the flange. The appearance of a series of invaginated 

 oblique cones is more striking in this species than in any other. 

 Fenestrated expansion two cm. or less in width, forming an 

 angle of from 55 to 60 with the axial line. On the reverse 

 the branches are narrowly rounded, from 23 to 25 in one cm., 

 near the axis as wide as the fenestrules, becoming narrowed 

 and somewhat zigzag toward the outer margin; fenestrules sub- 

 quadrate, sometimes perceptibly pentagonal or hexagonal; 16 

 or 17 in one cm.; dissepiments nearly as strong as the branches. 

 On the obverse the branches are usually without a keel, but a 

 few spines may be detected. Zooecia in two ranges, about 26 

 in five mm., with small aperture and peristome. On this side 

 the fenestrules are narrower and elliptical. 

 Diam. of shaft of strongest ex. 4.5 mm.; No. of vol. in 2 cm., 4.2. 



" " smallest " 2.3 " " " " 4.5. 



" " average " 3.1 " " " " 4.0. 



The larger size, less numerous volutions, and the compara- 

 tively greater extent of the flange, as well as the absence of a 

 spiniform keel and the smaller zooecia, distinguish this species 

 from A. proutanus, to which it is closely related. A. intermedius 

 Ulrich, might be confounded with it, but a careful comparison 

 will show among other differences, that while in that form the 

 frond diverges from the axial line at an angle of about 72, it 

 takes place in this species at an angle of only 55 to 60 



Position and locality: Chester group, Chester, 111. 



ARCHIMEDES PROUTANUS Ulrich. 



PI. LXIII, figs. 3-3d, and 11, lib. 



Axis slender, as usual both sinistral and dextral, very gradu- 

 ally diminishing toward each extremity, appearing generally as 

 of uniform size throughout. The longest specimen seen is of 

 the average thickness, nearly complete, and ten cm. long. Volu- 

 tions very regular, but varying in different examples of the typi- 

 cal form from 6 to 8 in two cm. Six examples before me belong 

 apparently to a variety of the species. Two of these are repre- 

 sented by figures 11 and lib. These differ from the typical form 

 principally in being stronger and in having from 4.5 to 5.5 



