340 ASSINNIBOINE AND SASKATCHEWAN EXPEDITION. 



Ammonites Placenta. 



[Ammonites placenta. — Dekay, N. Y. Lyc. Nat. Hist., 

 vol. ii. pi. v, fig. 2 ; Journ. Acad. Nat. Sci., Phila., vol. vi. 

 p. 88, &c. ; Morton, Synop. Org. Bern., p. 36, pi. ii. figs. 

 1 and 2. 



Locality and position. — South Branch of the Saskat- 

 chewan, from an equivalent of Formation No. 4 of the 

 Nebraska Cretaceous series.] 



FOEMATION NO. 5 OF VEETICAL SECTION. 



This very interesting bed, though differing lithologically 

 from the preceding one, contains many of the same 

 species of fossils. It is worthy, however, of a distinct 

 position in the series, not only from its extent, thickness, 

 and difference of composition, but also from the more 

 favourable influence that it exerts upon the country un- 

 derlaid by it. In ascending the Missouri Eiver it first 

 makes its appearance near the mouth of Grand Eiver, 

 about 150 miles above Fort Pierre. Near Butte aux Gres 

 it becomes quite conspicuous, acquiring a thickness of 

 eighty or one hundred feet, and containing great quan- 

 tities of organic remains. Here it forms an extension of 

 what is called Fox Eidge, a series of high hills having a 

 north-west and south-east course, crossing the Missouri 

 Eiver into Minnesota at this point. Its north-eastern 

 limits I have not ascertained. In its south-western ex- 

 tension it continues for a considerable distance nearly 

 parallel with the Missouri, crosses the Moreau Eiver 

 about thirty miles above its month, then forms a high 

 dividing ridge between the Moreau and Sheyenne Eivers, 

 at w T hich locality it first took its name. Continuing thence 

 its south-westerly course, it crosses the Sheyenne, and is 

 seen again in its full thickness at the heads of Opening 



