482 UNITED STATES GEOLOGICAL SURVEY OP THE TERRITORIES. 



is rather larger than the siphonal lohe, a little oblique, narrow at its base, 

 and very deeply divided into two equal, bifurcating, variously subdivided, and 

 digitate branches. The first lateral lobe is very large, and has at the extrem- 

 ity, two great spreading, unequal branches, each of which bifurcates twice, 

 and is provided with various subordinate divisions, with sinuous and 

 serrated margins. The second lateral sinus is larger and more erect than the 

 first, but similar in its mode of branching. The second lateral lobe is 

 about one-fourth smaller than the first, and divided in much the same way; 

 while the third lateral sinus is smaller than the first, very oblique, narrow at 

 its base, and deeply divided at the extremity into two nearly equal, sinuous 

 branches. The antisiphonal lobe is comparatively large, being about half 

 the size of the siphonal lobe, and tripartite at the extremity, with a tridentate 

 middle division. 



The greatest transverse diameter of the whorl is 2.43 inches; breadth 

 of umbilical cavity, 0.65 inch ; diameter of larger end of the whorl, 1.04 

 inches; diameter of the smaller end, 0.74 inch ; diameter of siphuncle at its 

 larger end, 0.06 inch. 



This species will be distinguished from all the otherwise similar forms 

 known in these rocks by the smaller size of its umbilical cavity, in propor- 

 tion to the diameter of its whorls, and by its more rapidly ascending spiral 

 curve. Its volutions are also more tapering, and differ in their peculiar 

 twisted character, indicated by the curve of the siphuncle ; there are like- 

 wise corresponding differences in the details of the septa. If it really 

 belongs to the genus Heteroceras, it would seem to be from the base of the 

 spire, just where the deflection of the body-part was commencing. 



Locality and position. — Great Bend of the Missouri below Fort Pierre, 

 Dakota ; from the lower part of the Fort Pierre group of the Upper Mis- 

 souri Cretaceous series. 



Heteroceras? anbilieatum, M. & H. 



Plate 22, fig. 5. 



TurrUitesl umbilicatits, Meek and Haydeu (1858), Proceed. Acad. Nat. Sci. Philad., VIII, 56. 

 Helicoceras umbiticatum, Meek and Haydeu (1860), ib., 185. 



Our specimen of this species consists of one entire volution, about half 

 of which is septate, though not in a condition to show the form and details of 

 all the lobes. I-t belonged to a sinistral shell, the volutions of which are 

 rounded, very gradually tapering, and coiled in an ascending spiral, so as at 



