504 . PALEONTOLOGY OF ILLINOIS. 



Angle of divergence, measuring along the narrower sides, 11°. Annular 

 striae 8 in .20 inch at the larger end, and 9 or 10 in the same space at 

 the smaller end. The specimen figured shows only the septate portion 

 of the shell. 



The most marked character of this species is its very regularly 

 arranged, equal stria?, which seem to pass almost, if not quite, directly 

 around the shell. They appear to be simple, uninterrupted and every- 

 where arranged their own breadth apart. It differs from 0. Laphami, 

 from the same rock, in its much more rapid expansion from the smaller 

 to the larger extremity, and in its compressed instead of cylindrical 

 form, as well as in having its striae passing directly around, instead of 

 obliquely. 



Position and locality : Joliet, Illinois ; Niagara Group, Upper Silurian. 



Orthoceras medullare, Hall? 



PI. 26, Fig. 1. 



Orthoceras medullare, Hall, 1860. Rep. Prog. Geol. Survey Wisconsin, p. 4. 20 Regents' Eept. N. Y. 



State Cab., p. 412, PI. 20, fig. 1 and 2. 

 O. stri&lineatum, McChesney, 1861. New Paleozoic Fossils, p. 94. 



Position and locality : Niagara limestone, Joliet, Illinois. 



Orthoceras angtjlattjm, Wahl. 



PI. 24, Fig. 8. 



Orthoceras angulatum, Wahlenberg, 1827. Nova. Acta. Sci. TJpsal.. p. 90. 



O. angulalum, Hall, 20 Regents' Report, p. 413, PI. XIX, figs. 10 and 11, and PI. XXIV, fig 



Position and locality : Niagara limestone Joliet, Illinois. 



Orthoceras rectum, Worthen. 



PI. 26, Fig. 3. 



Shell of medium size, very gradually tapering, septa moderately con- 

 cave, two of the intervals being a little less in width than the diameter 

 of the shell. Length of specimen with twelve septa preserved, 8.87 

 inches, length of outer chamber about 3 inches. Surface markings and 

 siphuncle unknown. 



This shell seems to be nearly related to O. crebescens of Hall, but dif- 

 fers from that species in its much less tapering form, and in the propor- 

 tional width of the septa. 



Locality and position : Joliet, Illinois, in the Niagara limestone, 

 Upper Silurian. 



