GUELPH FAUNA IN THE STATE OF NEW YORK 79 



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

 but differs from that species in its much less tapering form, and in the 

 proportional width of the septa. 



Worthen gives "the Niagara limestone " of Joliet 111. as horizon and 

 locality of his species. 



Before us is a fragment retaining 10 camerae from the lower Shelby 

 bed, which differs from the associated O. crebescens in the same 

 respect as indicated above for O. rectum. It expands so slowly that its 

 apical angle is only about 5, and the camerae are so deep that the 

 diameter is but a little larger than the combined depth of three camerae. 

 While the latter are therefore still somewhat shallower than those of the 

 type specimen of O. rectum, the difference is so small that it is well 

 within the limits of individual variation. The section of the Shelby species 

 is subcircular, and the siphuncle is centren. 



In the same year in which Worthen differentiated O. rectum from 

 O. crebescens, Hall and Whitfield described and figured a specimen 

 from the "limestones of the Niagara group" at Cedarville O., which they 

 referred to O. crebescens, but which, in the differential characters cited 

 above, tallies quite closely with the lower Shelby specimen. Hall and 

 Whitfield also noted the difference in the depth of the chambers between 

 the Ohio and the original Wisconsin specimens of O. crebescens, but 

 did not consider it sufficient for separation. The additional difference in 

 the rate of expansion brought out by the figure of the Ohio specimen and 

 its description, in which it is said to taper " gradually and moderately," 

 while the types of O. crebescens are stated to taper " rapidly," seems 

 to have been overlooked in the comparison of the forms. The constancy 

 of the combined appearance of these differences in the Illinois, Ohio and 

 New York specimens and the very distinct habit resulting from them, fully 

 warrant their recognition as of specific importance. 



