CEPHALOPODA. 421 



impressions upon the cast, the surface is shown to have been marked by fine, 

 lamellose, somewhat undulating striae of growth, and finer elevated lines, 

 which are interrupted by coarser revolving striae, and tbese are especially 

 conspicuous on the dorsal side, on the margins, and within the cavit}' of the 

 umbilicus, forming low rounded ridges, which are at unequal distances. 

 Similar, but less conspicuous, revolving striae or low ridges, are often pre- 

 served over the ventral face of the shell, while a single specimen preserves 

 the marks of obsolescent, low undulations, as in some specimens of N. subli- 

 ratus. In another specimen, near the aperture, there is a belt more than 

 twenty mm. wide, which is strongly marked by rounded, transverse undu- 

 lating striae, the undulations being caused by slender revolving striae, of 

 which there are six or seven in the space of five mm. 



The internal cast is essentially smooth and polished (this feature being due 

 to the nature of the matrix), with the exception of the suture lines, the 

 impressions of the transverse striae, and the obscure radiating striae, and 

 obsolescent undulations. The largest individual observed has a diameter of 

 about 200 mm., as measured across the volutions at the base of the grand 

 chamber; the entire individual has been much larger. 



This species has its nearest relations with N. maximus, and possesses many 

 features in common with that form. With a single exception all the specimens 

 are laterally compressed, and this may conduce to the contraction of the 

 umbilicus and the apparent re-entrant character of the volutions which is 

 observed in comparison. The specimens are preserved in a soft, fissile black 

 shale, and the surface-markings are more distinctly retained upon the cast, or 

 upon the adhering inner laminae of the shell, than in N. maximus, which is 

 usually imbedded in a coarser material. The pitted and mammillary deposit, 

 which everywhere marks the exfoliated specimens of N. maximus, is either 

 entirely absent, or scarcely at all observable on the specimens of N. oriens. 



This form is distinguished from N. magister by its comparatively wider 

 umbilicus, the greater exposure of the inner volutions, the nearly circular 

 transverse section, and sinus in the aperture. The surface-markings and the 

 absence of nodes on the ventro-lateral margins are distinctive features. 



