INVERTEBRATES. 163 



volutions slender, increasing very gradually in size, a little 

 broader transversely than the diameter from the dorsum to 

 the ventral margin, somewhat ventricose and rounded on 

 each side, and provided with a deep rounded sulcus along 

 each dorso-lateral margin. Between these two sulci the 

 dorsum is narrow, prominent, and less deeply concave, the 

 sulcus being bounded on each side by an angle. Sur- 

 face retaining traces of rather strong, regular, longitudinal 

 lines. Septa deeply concave, moderately close — their margins 

 arching distinctly backwards in each of the dorsal depressions. 

 Siphuncle rather small, and placed a little nearer the dorsal 

 than the ventral side. Greatest diameter of a specimen retain- 

 ing about one-third of the outer chamber, 2.23 inches; breadth 

 of last whorl, about 0.87 inch. 



This species bears ' some resemblance to certain varieties of JVi sulcatus, of 

 Sowerby, as figured by deKoninck (An. Foss., pi. XL VII, fig. 10, and pi. 

 XLVIII, figs. 8 and 9), but differs in the number and arrangement of its re- 

 volving angles, and in the possession of longitudinal striae. It is still more 

 nearly allied to iV". Edwardsianus, de Koninck (Sup. An. Foss., pi. LIX, 

 fig. 7 a, b, c,), but differs in having less compressed whorls, which are rounded 

 instead of angular around the umbilicus; while its dorsal carinas are less promi- 

 nent. Our specimen being merely a cast, we have no means of determining 

 how near these two forms may have agreed in the details of their surface 

 markings. 



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

 derhook Group, Subcarboniferous series. 



Nautilus digonus, M. and W. 



PL 14, Fig. 9 a, 9 6, 9 c, 9 a". 



Nautilus (Discus) digonus, Meek and Worthen, Oct., 18C0. Proc. Acad. Nat. Sci., 

 Philad., p. 470. 



Shell small, subdiscoidal ; umbilicus comparatively large, 

 showing all of each inner whorl, apparently perforate ; volu- 

 tions slender, increasing very gradually in size, a little broader 



