INVERTEBRATE PALEONTOLOGY. 470 



the same size and general form on opposite sides of the siphuoele. The 

 siphonal lobe is comparatively small, and ornamented id (lie extremity by 

 four small branches, the two terminal of which are a little larger 1 hau the 



others, slightly dissimilar, and each provided with five or six unequal, sharp 

 rligitations; the other two branches arc not exactly opposite, differ slightly 

 in form, and are each armed with from three to five or six unequal serra- 

 tions; in advance of these principal terminal divisions, there arc, along the 

 sides of the lobe, a few very small, alternating, subordinate, lateral branchlets 

 and sinuosities. The first lateral sinus is small, very oblique, much con- 

 tracted at its base, and divided at the extremity into two unequal, variously- 

 subdivided, sinuous branches. The first lateral lobe is much larger than the 

 siphonal one, and very deeply divided into two great spreading, subequal 

 branches, the larger of which is on the ventral side, and unequally subdivided 

 into three bifurcating branchlets, the two terminal of which are much larger 

 than the others, and more or less digitate, while the other main branch has 

 two principal bifurcating branchlets, with many smaller digitations and sinu- 

 osities. The second lateral sinus is not so oblique, but in other respects 

 very similar to the first ; while the second lateral lobe is smaller than the 

 first, and very much like it in its mode of branching. 



The largest and best specimen of this species that has been found, con- 

 sists of a little more than the- half of one volution, the greatest transverse 

 breadth of which is '2.3A inches ] diameter at larger end (which is a little 

 oval), 0.73-by 0.64 inch; breadth of umbilical space, 0.85 inch. 



Since writing the foregoing description, the only specimen of this spe- 

 cies that we had, showing the septa, was lost, consequently 1 am unable to 

 give an enlarged view of its lobes and sinuses as was intended. It will 

 be seen, however, from the description, that in their general characters they 

 are like those of Helicoccras Mortoni, H. & M. 



In form and other external characters, even fragments of this shell will 

 be at once distinguished from the H. Mortoni, by its shorter curve and pro- 

 portionally thicker whorls, which also differ in being nearly or quite in con- 

 tact. Its costae are also smaller in proportion to the diameter of the whorls, 

 and more regular. 



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

 Dakota ; from the lower part of the Fort Pierre group. 



