CRETACEOUS FOSSILS. ■ • 455 



branches. There are also two lateral dentate spurs above on each side of 

 the lobe. First lateral lobe large but slender, divided into two principal 

 widely-divergent branches, each of which is again divided, and furnished 

 with several strong dentate spurs on their margins; two other spurs of 

 similar character exist on each side of the lobe, one just below the bifurca- 

 tion and the other some distance above. Second lateral lobe smaller than 

 the first, deeply divided, with the branches less divergent, but again 

 divided, and the inner branches trifurcate, the outer branches short and 

 strongly dentate. Antisiphonal lobe small and simple, marked by one 

 small spur on each side above and two larger curved and dentate ones 

 below; the central termination being minutely tridendate at its extremity. 

 First lateral sinus large, very deeply divided into two principal branches, 

 each of which is deeply divided above and strongly lobed on the sides. 

 Second sinus smaller, less deeply and unequally divided, the outer half 

 smallest and deeply lobed, the other less deeply so. Antisiphonal sinus 

 very large (considering the parts separated by the antisiphonal lobe as one 

 sinus), the antisiphonal lobe dividing it nearly to the base, and the divisions 

 again very deeply divided and deeply lobed on the sides. Siphuncle small, 

 situated between the lines of nodes and marginal. 



This species differs from Ancyloceras Jennyi, in being sinistrally coiled, 

 and in the character of the ridges of the surface and angular nodes. We 

 know of no other species described with which it is enough related to 

 require a comparison. 



Formation and locality. — In limestone at the top of the Fort Pierre 

 Group, on the Cheyenne, near French Creek, Black Hills. 



Genus PTYCHOCERAS D'Orb. 



In his remarks on the genus Ptyclioceras (Paleontology of the United 

 States Geological Survey of the Territories, p. 410), Mr. F. B. Meek has 

 advanced the opinion that these shells have possibly been folded upon 

 themselves a second time in their more advanced stages of growth, as in 

 the case of Diptychoceeas Gabb; or that they may have even been bent 

 backward again, as is supposed to have been the case in the genus Soleno- 



