1G-4 PALAEONTOLOGY OF ILLINOIS. 



transversely than the diameter from the dorsum to the ventral 

 margin, nearly flat, and provided with two shallow longitudi- 

 nal grooves on the dorsal side, and distinctly angular around 

 each dorso-lateral margin, thence rounding regularly into the 

 umbilicus. Surface of cast retaining traces of regular, equidis- 

 tant, longitudinal lines, and much finer closely arranged trans- 

 verse stria?. Aperture and section of the whorls nearly semi- 

 circular, the outer or dorsal side being almost straight, and the 

 inner rounded. Septa distinctly concave on the side next the 

 aperture; their margins curving abruptly forward in crossing 

 the dorso-lateral angles, thence deflected backwards in the 

 dorsal grooves, and passing nearly straight across the middle 

 of the dorsum. Siphuncle small, located about one-third the 

 diameter of each septum from the dorsal side. Length, or 

 greatest diameter of a specimen, about one-third of the outer 

 whorl of which is non-septate, 1.25 inches; height, 1 inch; 

 breadth across the outer whorl, near the aperture, 0.62 inch; 

 diameter of same from the dorsal to the ventral side, 0.40 inch. 



Prof. Hall figures in the Thirteenth Ann. Report Regents University N. Y., 

 p. 105, under the name of Gyroceras gracile, a fragment of a shell apparently 

 related to this species. As he does not say, however, in his brief description, 

 whether it has rounded or angular whorls, and his figure is merely an outline 

 wood cut, we are left in some little doubt whether it may not be a fragment of 

 a variety of the species before us, especially as it came from the same locality 

 and position, and appears to agree in its surface markings. The measurements 

 of its proportional vertical and transverse diameters, however, would seem to 

 indicate narrower whorls and a more compressed form than in our shell, while 

 his figure represents the undulations of the septa in crossing the dorsum as dif- 

 fering somewhat from those of our species. If they should prove to be identi- 

 cal, however, our name will have to take precedence, since our description was 

 published more than a month in advance of Prof. Hall's. Even if specifically 

 distinct, we have little doubt in regard to the shell described by Prof. Hall be- 

 longing to the same group as ours. 



Locality and position : Rockford, Indiana, in the Goniatite bed of the Kin- 

 derhook Group. Subcarboniferous. Also, at the same horizon in Jersey 

 county, Illinois, and at several places in Central Missouri. 



