INVERTEBRATES. 149 



margin of the shell. Septa moderately convex and oblique to 

 the axis, being arched strongly upward on the siphuncular side, 

 and equaling in width about one-fifth the shortest dianfeter of 

 the shell. Surface markings unknown. 



Length of the largest specimen seen, about 6 inches; greatest 

 width at the apex % inch ; diameter of outer chamber 2% inches. 



Position and locality: Chester limestone, Chester, 111. 



Collector, A. H. Worthen. 



No. 2484 of the Illinois State Museum. 



ORTHOCERAS OKAWENSE. Worthen. 



PL XXVI. Fig. 3. 

 Orthoceras okawense, Worthen. 1883. Geol. Surv. of Illinois. VoL VII, page 324. 



Shell elongate, slender, very gradually tapering to the apex; 

 septa concave, about four of them in the space of one diameter. 

 Siphuncle sub-central; surface markings unknown. 



This shell has a general resemblance to O. rushense, Mc- 

 Chesney of the Upper Coal Measures, but differs from it in the 

 position of the siphuncle, which in our species is decidedly sub- 

 central. 



Position and locality : Chester limestone, near Red Bud, Ran- 

 dolph county, 111. 



Collector, A. H. Worthen. 



No. 2485 of the Illinois State Museum. 



ORTHOCERAS LASALLENSE. Worthen. 



PL XXVL Figs. 4-te. 

 Orthoceras lasallense, Worthen. 1883. Geol. Survey of Illinois, VoL Vli, page 324. 



Shell small, gradually tapering. Surface ornament with trans- 

 verse ridges or elevations, situated at regular distances from 

 each other, and with delicate thread-like striae which cover the 

 transverse ridges as well as the spaces between them. 



The ridges and striae will serve to distinguish this species from 

 any hitherto described from the Coal Measures, its ornamenta- 

 tion resembling a Silurian form, O. undulatum, of the Niagara 

 limestone, more closely than any other known American species. 

 The fragment preserved and figured seems to belong to the non- 

 septate portion of the shell. 



