500 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



like mass is much more probably no part of the shell itself, but merely a 

 portion of the same inorganic rocky material from which the specimen was 

 broken, filling a small umbilical opening of the outer volution. This sugges- 

 tion is also apparently sustained by an examination of the otber side of the 

 specimen, illustrated by fig. 2, c, of the same plate. Here it will be seen 

 that there is also, as it were, a little plug in the umbilicus. Most of this, 

 however, is clearly layers of shell ; but a careful examination of its broken 

 end on the right, seems to show that there is passing into it a minute umbili- 

 cal cavity, filled with rock so nearly of the same color as the partly-meta- 

 morphosed shell itself that it requires very careful looking to see the line of 

 separation between the two. On this side, however, as examined much 

 farther in than can be seen on the other, the filled cavity is so very small as 

 to show that the umbilicus must be completely closed in young and medium- 

 sized specimens ; while the apparently inorganic plug on the other side 

 seems likewise to show that there is a small, deep umbilical opening in the 

 outer volution of the adult shell. 



From these facts, I am led to believe that our shell will be found to 

 agree so closely with Sowerby's species that there may be no necessity for 

 separating it, even as a variety — that is, if Mr. Sharpe's illustrations can be 

 relied upon. From Sowerby's application of the words " indistinctly sagit- 

 tate," however, to the aperture, it would seem that his type-specimen must 

 be much more compressed than ours, which, as already stated, agrees well 

 with Sharpe's figures in form.* As Sharpe ought to have been well acquainted, 

 however, with Sowerby's species, I infer that the latter' s original type may 

 have been accidentally compressed. 



On comparing our shell with the form figured by d'Orbigny in the 

 Palaeontology of France, and referred by him to N. elegans, Sowerby, it will 

 be seen that the latter is also rather decidedly more compressed, and has its 

 siphuncle a little farther from the middle of the septa ; while he describes its 

 umbilicus as being not open, but only provided with a slight depression. 



These differences, or more properly the position of the siphuncle, has 

 led Pictet, in his review of the costate forms of Nautili, to regard IV. elegans 

 of Sowerby, and N. elegans of d'Orbigny, as belonging to two distinct species ; 

 and this conclusion has also been adopted by Blanford in his report on the 



* Sowerby says that the cross-diameter of bis species about equals two-thirds of its height; but 

 I am not sum iu regard to how he applied the words " cross-diauieter" and " height " ; and his singlo 

 tigure being an oblique view does not show the form of the aperture. 



