Logan.] Tne Invertebrates, Fort Pierre Group. 513 



unequal serrations ; in advance of these principal terminal 

 divisions, there are, along the sides of the lobe, a few very 

 small, alternating, subordinate, lateral branchlets and sinu- 

 osities. The first lateral sinus is small, very oblique, much 

 contracted at base, and divided at the extremity into two 

 unequal, variously subdivided, sinuous branches. The first 

 lateral lobe is much larger than the first siphonal one, and very 

 deeply divided into two great, spreading subequal branches, 

 the larger of which is on the ventral side, and unequally sub- 

 divided into three bifurcating branchlets, the two terminal of 

 which are much larger than the others, and more or less digi- 

 tate, while the other main branch has two bifurcating branch - 

 lets, with many smaller digitations and sinuosities. 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 consists of a little more than half of one volution, the 

 greatest transverse breadth of which is two and thirty-four 

 hundredths inches ; diameter at larger end (which is a little 

 oval) , seventy-three hundredths by sixty-four hundredths inch ; 

 breadth of umbilical space, eighty-five hundredths inch. In 

 form and other external characters, even fragments of this shell 

 will be at once distinguished from H. mortonii by its shorter 

 curve and proportionally thicker whorls, which also differ in 

 being nearly or quite in contact. Its costse are also smaller in 

 proportion to the diameter of the whorls, and more regular." 



The determination of this species as belonging to the Fort 

 Pierre fauna of Kansas is based upon a single fragment imbed- 

 ded in a hard, nodular mass of stone. The specimen was col- 

 lected from the lower Fort Pierre shales of Butte creek, and is 

 associated with I. alius. 



