388 PALEONTOLOGY OF ILLINOIS. 



Keyserling, to N. tuberculatum, Sowerby, in their Gcol. Russ. and Ural Mount., 

 vol. ii, pi. xxv, fig. 12 a, b, but differs in having its sides flattened and sloping 

 outwards from the prominent margins of its umbilicus, instead of inwards from 

 the outer angles; while the nodes of its outer angles are more numerous and 

 more crowded. According to Sowerby's and Phillips's figures of N. tubercu- 

 latum, the node-bearing angles are situated near the middle of each side, from 

 which points the whorls round abruptly into the umbilicus, without any inter- 

 mediate flattening of the sides. Hence our shell must differ more widely from 

 the form upon which N. tuberculatum was founded, than from the Russian form 

 alluded to. Prof. McCoy also describes N. tuberculatits as having the "siphun- 

 cle very large, central," while in our shell it is small and placed a little outside 

 of the centre. 



The shell figured by Dr. Owen as Discites tuberculatum, Owen, (Report Geolo- 

 gical Survey Wisconsin, Iowa and Minnesota, tab. V, fiig. 14), is possibly iden- 

 tical with our species, but as Sowerby had used the same name for an allied 

 species of this group, we cannot retain it for this shell. 



Locality and position: Upper Coal Measures, Sangamon county, Illinois. • 



Genus CYRTOCERAS, Goldfuss, 1832. 



(Dech. in de la Beche, p. 536.) 



Cyrtoceras (Aploceras) curtum, M. and W. 



PI. 30, figs, la, 16, lc. 



Cyrtoceras curtum, Meek and Worthen, October, 1860. Proceed. Acad. Nat. Sci., 

 Philad., p. 468. 



Shell having the form of a short, moderately compressed, 

 rapidly tapering, and slightly curved cone; section subcircular 

 a little below the middle, but transversely oval above, — the 

 dorsal and ventral sides being compressed, convex, and the 

 lateral margins somewhat flattened. Surface ornamented by 

 numerous, regularly arranged, annular striae, or impressed lines, 

 which arch a little forward in crossing the dorsum, where they 

 are separated by spaces several times their own breadth, 

 excepting near the smaller extremity of the shell. On the 

 lateral and ventral sides these striae become much more crowded, 

 and more deeply impressed. Septa rather deeply concave ; 

 siph uncle small, and placed about half way between the middle 

 and the dorsal side. Length of a specimen imperfect at the 



