380 GEOLOGY OF THE BLACK HILLS. 



the septa and in the character of the umbilicus when compared with 

 specimens of the same size that would seem to distinguish them. A. cor- 

 clatus does not appear to become so ventricose, nor are the volutions so 

 nearly triangular in the larger individuals as in the western shells. 



Formation and locality. — In rocks of Jurassic age, associated with other 

 Jurassic forms, at a horizon 350 feet above the red beds referred to the 

 Triassic, at Red water Valley; also on the tops of the highest hills two miles 

 south of the Belle Fourche River, near Bear Lodge Butte, Black Hills. 



AMMONITES OORDIFOBMIS var. DISTANS. 



Plate 6, fig. 25. 



Shell of moderate size, slightly cordiform in a transverse section of the 

 volution, or approaching subquadrate, and distinctly keeled on the back. 

 Volutions few in number, largely embracing, leaving a broad, open umbili- 

 cus equal to nearly one-third of the diameter of the entire shell, and in 

 which is exposed less than one-third of the breadth of each volution. Um- 

 bilical margin of the volution abruptly rounded, sides depressed convex, 

 somewhat flattened in the middle, slightly decreasing in convexity and 

 thickness outward to about the outer fourth of the width, beyond which 

 point the surface abruptly declines to the thickened, rounded, and promi- 

 nent dorsal keel. 



Surface marked by strong, distant, and abruptly elevated flexuous ribs 

 or ridges, with concave interspaces; originating a little outside of the umbil- 

 ical margin, they are directed, with a slight sigmoidal curvature, nearly 

 across the volution, and are then directed abruptly forward along the dorsal 

 slope for a distance nearly equal to one-half the width of the volution at the 

 point of their occurrence, and form strong, wave-like ridges in crossing the 

 dorsal carina. On the dorsal slopes there are also one and sometimes two 

 additional ridges between the primary ones, but usually of a little less 

 strength. The primary ridges sometimes form pointed spine-like nodes at 

 their intersection with the intermediate or secondary ones ; but this is not 

 a constant feature, even on the same shell, and only occurs when the pri- 

 mary ridge dies out or bifurcates, forming two secondaries on the slope, 



