204 



NORTH AMERICAN INDEX FOSSILS. 



356. H. simplicostatum (Whitfield). (Fig. 1472.) Cretacic. 



Spiral much looser than in preceding; ribs more numerous, 



sharper and separated by concave interspaces ; tuberculated in old 



Fig. 1470. Helicoceras stevensoni, X /^- (After Whitfield, Pal. Blk. Hills.) 



age; section circular except last part. (For a full description and 

 illustration of this remarkable form and its ptychoceran stage, 



Fig. 147 1. Helicoceras stevensoni, 

 suture enlarged, X |- (After Whit- 

 field, Pal. Blk. Hills.) 



Fig. 1472. Helicoceras simplicosta- 

 tum, X /^. fragment. (After Whit- 

 field, Pal. Blk. Hills.) 



see Whitfield, Bull. Am. Museum of Natural History, Vol. 16, pp. 

 68-72, pis. 23-27.) 



Benton group of Black Hills, etc. 



