CEPHALOPODA. 427 



regularly and gradually enlarging. Apical angle about 7^° on the convex 

 ventral side, and 9° on the lateral faces. 



Chamber of habitation large, not ventricose, occupying fully half a volution; 

 becoming free from the adjacent volution near the aperture. Aperture, as 

 inferred from the direction of the lines of growth, quite oblique, opening 

 outwardly, with a very broad, deep sinus on the ventral side, and a gentle 

 sinus on the lateral margins. Air-chambers undetermined — the shell cover- 

 ing that part of the specimen. 



Septa, suture lines and siphuncle undetermined. 



Test thin. Surface marked by fine, irregular, lamellose lines of growth, 

 which are crossed by fine, sharp, irregular or undulating, thread-like, revolv- 

 ing striae, giving a cancellated surface. Tube ornamented by a row of strong, 

 rounded, slightly transverse nodes on the ventro-lateral angles ; and a row 

 of smaller and more slender transverse nodes on the dorso-lateral angles, 

 which descend slightly into the umbilical cavity. The nodes on the umbilical 

 margin are twice as numerous as those on the ventro-lateral angles. 



Greatest diameter of the specimen about 100 mm. 



This species, in its general features, resembles N. (Discites) Marcellensis of 

 Vanuxem, but is distinguished by the quadrangular transverse section, with 

 the sides flat or concave, and by the convex ventral side, which is wider than 

 the dorsal side. It has likewise a smaller apical angle and different surface 

 characters. In the proportions of width on the ventral and dorsal sides it is 

 the reverse of N. (D.) Marcellensis, which is narrow and flat on the ventrum. 

 The surface striae are very similar in the two species ; but the presence of 

 a row of small, prominent nodes on the umbilical margins of this one is a 

 strongly distinctive feature. The species is extremely rare, so far as known, 

 but a single specimen having come under observation. 



Formation and locality. In limestone of the Upper Helderberg group, near 

 Sandusky, Ohio. 



