452 GEOLOGY OF THE BLACK HILLS. 



The septa are arranged at considerable distance from each other, leav- 

 ing an open space between. The convolutions of the lobes and sinuses are 

 very complicated, and are divided and branched much as in other species 

 of the group from the same locality. The details of their structure may be 

 seen by reference to the diagram on the plate. Siphuncle of moderate size 

 and situated just above the middle of the volution 



The species differs from either of the related forms here described in the 

 proportional elevation of the spire and the compactly-coiled volutions, being 

 much less elevated than Helicoceras stevensoni, and more elevated than Hete- 

 roceras newtoni. The tube also increases more gradually than that of the 

 latter species, and forms a smaller umbilical opening. They also differ 

 materially in the details of the septa. The specimen figured, as far as pre- 

 served, shows no evidence of the deflection of the outer volution ; but the 

 compact volutions would seem to indicate relations with Heteroceras rather 

 than with Helicoceras. 



Formation and locality. — In limestone on the west side of the east fork 

 of Beaver Creek, Black Hills ; supposed to belong to the Fort Pierre' Group 

 of the Cretaceous section, but which also contains a mingling of forms of 

 No. 5 of the same section. 



Genus ANCYLOCERAS D'Orb. 



ANOYLOCERAS JENNEYI. 



Plate 16, figs. 7-9. 

 Ancyloceras Jenneyi Whitf., Prelim. Rept. Pal. Black Hills, 1877, p. 42. 



Shell of moderate size, composed of two or more (?) rapidly-increasing 

 volutions, enrolled on the same plane, and entirely disconnected, the inner 

 ones apparently leaving a broad, open, umbilical space. Shell rather rap- 

 idly increasing in size with increased age. Volutions slightly flattened on 

 the back, vertically compressed, and transversely oval in section; the ver- 

 tical diameter being only about two-thirds as great as the dorso-ventral in 

 the larger parts of the shell, the greatest height being nearest to the ventral 

 margin. 



