304 PALEONTOLOGY OF ILLINOIS. 



the middle and the suture at the base of each of those compos- 

 ing the spire. Surface marked by numerous fine, regular 

 thread-like revolving lines, crossed by similar strise, which in 

 traversing the spiral band, make a slight backward curve 

 parallel to the margin of the sinus in the lip. Length, 0.75 

 inch; breadth, 0.72 inch; apical angle regular, divergence 

 60 °; breadth of spiral band, about 0.04 inch. 



This species resembles P. tabulata, of Conrad, but is smaller than the average 

 size of that shell, while it differs in having its spiral band located midway be- 

 tween the two carinae, instead of coincident with the upper angle of the body 

 whorl. It also wants the crenulations on the principal angle, so well marked 

 on that of P. tabulata, and is slightly umbilicated, instead of having an imper- 

 forate axis, as in Mr. Conrad's species. Again it differs in being much more 

 finely and more regularly cancellated over the entire surface ; being in this 

 respect much more like the species we have referred to, P. Brazoensis, of Shu- 

 mard, from which it will be readily distinguished by its other characters. 



Locality and position : Chester, Illinois ; Chester group of the Lower Car- 

 boniferous series-. 



CEPHALOPODA. 



Genus ORTHOCERAS, Auct. 

 Orthoceeas annulato-costatum, M. and W. 



PI. 24, figs. 3 a, 3 b. 



Orthoceras annulato-costatum, Meek and Worthen, June, 1861. Proceed. Acad. Nat. 

 Sci., Philad., p. 147. 



Shell attaining a medium size, and having the form of an 

 elongated, moderately compressed cone, the sides of which con- 

 verge towards the apex at an angle of about 14°. Section 

 elliptical, the greater transverse diameter being to the smaller 

 as 100 to 80. Surface ornamented with slightly oblique, annu- 

 lar costae, which are less than the depressions between, and 

 rather sharply elevated on the smaller half of the shell, but 

 become gradually obsolete towards the aperture, where they 



