436 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES- 



are not, it must be confessed, very important, or even not greater than we 

 may frequently see between those of different individuals of the same spe- 

 cies ; but it must be remembered that we do sometimes see the septa of 

 species clearly distinct, as regards all other characters, quite as nearly alike 

 as these. A more important difference, however, is the entire absence of 

 tubercles on the sides of all the volutions, both inner and outer, of the form 

 under consideration ; while in the last they are well developed on the inner 

 turns, as well as on a part of the outer one. Again, it will be observed that 

 the costse are proportionally smaller and more crowded on all of the inner 

 volutions of the form here described than in the last, which is also decidedly 

 more convex. 



Our specimens of this shell agree in nearly all respects with Dr. Mor- 

 ton's figure of his A. Nicolletii, as well as with Dr. Owen's figures cited at 

 the head of this description, and I cannot doubt that they all belong to the 

 same species. 



It will be observed that this shell departs from the Discoscaphites group 

 in one rather prominent character; that is, in the entire absence of tubercles 

 on the sides of its volutions. From its close general relations to the last, 

 however, in which that character does occur, both in form and in the details 

 of its septa, it seems improper to place it in any other section. 



Among . foreign species, it may be compared with varieties of S. con- 

 strictus, Sowerby (sp.), as illustrated by d'Orbigny (Pale"ont. Fr., Terr. Cret, 

 I, pi. 129, fig. 8), which it nearly resembles, though its last turn seems never 

 to become so free at the aperture, and the deflected part of its body-volution 

 wants the broad costse or undulations seen on that shell, which also has more 

 prominent nodes around the angles of its periphery. The lobes and sinuses 

 of the septa in these shells are also very similar, except in details Our 

 shell, however, does not present the varieties of form illustrated by d'Orbigny's 

 figures of S constrictus, being very constant in general form, and the breadth 

 (height) of the deflected part of the last turn. Possibly, it may not be distinct 

 from Sowerby's species; but, without specimens of that shell for compari- 

 son, I do not feel warranted in adopting that conclusion. 



Locality and position. — Moreau River, Fox Hills, Long Lake, &c, 

 Dakota, and Yellowstone -River, Montana ; in the Fox Hills group of the 

 Upper Missouri Cretaceous series. Dr. Morton's type-specimens were from 

 Dakota, but the exact locality is not given. 



