574 PALEONTOLOGY OF ILLINOIS. 



occurs at most localities in the group, but is especially abund- 

 ant at Sloan's Valley, Pulaski Co., Ky. 



ARCHIMEDES INTERMEDIUS Ulrich. 



PL LXIII, flg. 2c. 



This form, which is also rather abundant, closely resembles 

 both A. communis Ulr. and A. swallovanus Hall. It differs from 

 the former in having from 5% to 6% volutions in two cm., and 

 in having a stronger axis. The angle formed by the fenestrated 

 expansion is also smaller, being about 72 degrees, while in A. 

 communis it is not less than 85 degrees. The last comparison 

 holds good also for A. swallovanus. The axis of Hall's species 

 makes from 4% to 5 volutions in two cm., and is generally also 

 much stronger. 



The Kentucky examples usually make one-half a volution 

 more in two cm. than the Illinois specimens. The section 

 figured was taken from an example from Chester, 111., which 

 had 6% volutions in that distance, and is an exception to the 

 .rule. 

 Thick, of shaft of strongest ex., 4.5 mm.; No. of vol. in 2 cm., 6. 



" smallest " 2.0 " " " 5%. 



" " average " 3.0 " " " " . 6. 



Position and locality: Chester group. Chester, 111., and 

 Sloan's Valley, Ky., and other localities. Common. 



ARCHIMEDES SWALLOVANUS Hall. 



PL LXIII, flg. 12-12d. 

 Archimedes swallovana Hall, 1857. Proc. Amer. Asso. Ad. Sci. vol. 10, p. 178. 



Axis rather strong, of considerable length, approximately of 

 the same diameter throughout (i. e. not appreciably fusiform); 

 volutions regular, 4% to 5 in two cm.: shaft strong but deeply 

 .concave, generally forming a regular curve, sometimes a little 

 straightened above, or again quite straight at the sides (see 

 fig. 12a). Fenestrated expansion of unknown width, forming 

 an angle of about 85 with the axial line. Branches 22 in one 

 cm., with a scarcely. perceptible simple keel, and two ranges of 

 zooecia; in each 19 or 20 occur in five mm. Fenestrules ellipti- 

 cal, twice as long as wide, about 14 in one cm. 



