498 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



tions, somewhat as in N. elegans and N. pseudo-elegans, never seen on N. 

 Dekayi at any stage of growth. His figure also represents the siphuncle 

 proportionally more nearly central than in Dr. Morton's species. 



Locality and position. — Dr. Morton's original type-specimens of N. 

 Dekayi were from Monmouth and Burlington Counties, New Jersey, where 

 it occurs in the Greensand-Marls of the Cretaceous. It also occurs at. various 

 other localities in that State at the same horizon. Our figured specimens on 

 plate 27 came from Sage Creek and Yellowstone River, of the Upper Mis- 

 souri Cretaceous ; where they were found in the upper part of the Fort Pierre 



group. 



Nautilus Dekayi, var. Moiitauaensis. 



Plate 27, figs. 2, a, b, c, d, c. 



This shell agrees exactly with N. Dekayi in the nature of its closed 

 umbilicus, and does not differ materially in general form, being only a little 

 more compressed laterally, which also gives a shorter transverse diameter 

 and somewhat different outline to its aperture. The most important differ- 

 ence, however, is to be seen in the position of its siphuncle, which is very 

 nearly central, instead of being only about one-fifth the dorso-ventral diam- 

 eter of the septa from the inner side. A difference of this latter kind is gen- 

 erally regarded as specific, and I am much inclined to think that this shell 

 will be found specifically distinct from N. Dekayi. Still, Mr. Blanford, of 

 the Indian survey, refers to the single species N. Bouchardianus, d'Orbigny, 

 forms differing as much in the position of the siphuncle, as well as in other 

 respects, as this differs from the typical form of N. Dekayi, and still states that 

 all of the Indian forms shade together by imperceptible gradations in all of 

 these characters. 



For the present, I therefore place this shell as a variety of N. Dekayi, 

 though suspecting that it will be found to present sufficiently-marked and 

 constant differences to rank as a distinct species. 



Locality and position. — Yellowstone River, Montana, one hundred and 

 fifty miles from its mouth ; where it occurs in beds containing a blending of 

 the fossils of the Fort Pierre and Fox Hills groups of the Upper Missouri 

 Cretaceous scries. 



