NATURAL SCIENCES OF PHILADELPHIA. 25 



more acutely conical umbilicus, and less broadly rounded dorsum. Its costae 

 are also more rounded, and it presents well marked differences in its septa. 



The only specimen of this species we have yet seen consists entirely of sep- 

 tate whorls, the non-septate outer portion having been broken away. It mea- 

 sures in its greatest, diameter 4 inches, and 257 inches in breadth at the widest 

 part of the outer whorl. 



Named in honor of Lieut. John Mullan, TJ. S. Top. Engrs., in charge of the 

 Expedition for the location and construction of the Pacific Wagon Road. 



Locality and position. Same as preceding. 



Genus NAUTILUS, Breynius. 

 Nautilus elegans, var. Nebrascensis, 



Nautilus elegans, SowerbyJ^816. Min. Conch., pi. 116. 

 Nautilus elegans, Mantell, 1822. Geol. Sussex, t. xx. fig. 

 Nautilus elegans, D'Orbigny, 1840. Palaeont. Franc. Ter. Cret., t. i. pi. 19. 

 Nautilus elegant, Sharpe, 1853. Foss. Mai. Chalk, pt. 1, Ceph. pi. 3, fig. 3, and pi. 

 4, fig. 1. Palaeontographical Society. 



Shell globose, broadly rounded over the dorsum and sides; umbilicus en- 

 tirely closed ; volutions increasing rapidly in size, considerably wider trans- 

 versely than from the dorsal to the ventral side; aperture transversely reniform 

 or subluuate, being deeply sinuous on the inner side for the reception of the 

 preceding turn ; margins of the septa rather abruptly arched forward near the 

 umbilicus, and slightly backwards on the dorsal side, deeply concave on the 

 outer side ; siphuncle located about its own breadth outside of the middle of 

 each septum. Surface of the body whorl ornamented by regular, flattened, 

 transverse costae about five times as broad as the grooves between. In crossing 

 over the dorsum, these costae all arch gracefully and deeply backwards parallel 

 to the broad dorsal sinus of the lip. On the inner whorls, the costse become 

 obsolete or are only represented by rather distinct lines of growth. 



Length or greatest diameter, 3-90 inches ; height, 2-82 inches ; breadth, 3-40 

 inches. 



This shell agrees almost exactly, in form and surface markings, with Mr. 

 Sharpe's figures of Sowerby's species, to which we have referred it, and only 

 differs in having its siphuncle a little more nearly central and its umbilicus 

 closed, apparently at all ages. According to Mr. Sharpe, the umbilicus of N. 

 elegans is closed in the young shell, but becomes a little open in the outer whorl 

 of large specimens. He also states that its siphuncle is located about half way 

 between the middle and the dorsal side of the septa, though in his'figure on 

 plate 4 of his Mouograph cited at the head of this description, it is represented 

 somewhat nearer the middle. Such small differences, howevtr, we can scarcely 

 regard as being of specific importance, but, if fuller comparisons should prove 

 our shell to be distinct, it can retain the name of Nebrascensis. 



It is worthy of note that D'Orbigny's figures cited above represent a rather 

 distinctly less ventricose form, with a more narrowly rounded dorsum than 

 our Nebraska shell, or those figured by Mr. Sharpe. He also shows a distinct 

 longitudinal line on the middle of the dorsum (of an internal cast) not seen 

 on our specimen, nor on those figured in Mr. Sharpe's Monograph. 



Dr. Shumard has described, in vol. i. p. 590 of the Transactions of the St. 

 Louis Academy of Sciences, a similar species from the cretaceous rocks of 

 Texas. As he mentions, however, that the siphuncle of the Texas shell is 

 situated between the middle and the ventral side of the septa, and that the 

 height of its aperture is greater than its breadth, he doubtless has a distinct 

 species from N. elegans. So far as we know, this is the first time Sowerby's 

 species has been even doubtfully identified in America. 



Locality and position.Sa.mQ as preceding. 



1862.] 



