448 GEOLOGY OF THE BLACK HILLS. 



volutions, sharply enrolled and leaving - an umbilical opening equal to or a 

 little greater than one-half the diameter of the volution encircling it. The 

 volutions are at first widely disconnected and rapidly descending, but 

 become more closely coiled below, and in the lower part of the example 

 figured are but slightly disconnected. 



Surface of the shell marked by strong, rounded, annulating costse, pass- 

 ing entirely around the shell, and separated by wider concave interspaces. 

 On the upper surface of the volution the costse are strongly directed back- 

 ward, but are recurved, and on the outer face are directed gently forward 

 below and within the umbilicus. The costse are further ornamented by two 

 irregular rows of strong, obliquely-rounded, tuberculose nodes ; the upper 

 range being situated along the middle of the volution, and the other half 

 way between it and the center of the basal surface. The nodes of the two 

 ranges, although usually placed on the same costa, are not constantly so, 

 but frequently alternate, and in very many cases two of the costas unite at 

 the lower node, continuing only as one on the lower side of the volution. 



Septa rather distantly arranged, leaving considerable space between 

 them, the convolutions of a single septum occupying a space on the periph- 

 ery of the shell equal to from two- thirds to three-fourths of the diameter 

 of the volution at the point examined. Siphonal lobe longer than wide, and 

 deeply divided at the extremity into two principal branches, each of which 

 is again deeply divided, and the divisions furnished with numerous ser- 

 rated digitations. First lateral lobe very large and unequally divided into 

 two principal branches, the outer one of which is the largest, and each is 

 again divided and furnished with numerous compound digitations. The 

 second lateral lobe is much smaller than the first and similarly divided, but 

 the divisions are smaller and less complicated (the inner half of this lobe 

 has not been distinctly traced). First lateral sinus rather large and deeply 

 cleft in the middle by an auxiliary lobe, the lower end of which is divided 

 into several digitations. Second lateral sinus shorter and comparatively 

 more spreading than the first, and also deeply divided in the middle by a 

 simple auxiliary lobe, having four spurs on each side. Other lobes and 

 sinuses not observed. Siphuncle rather small, marginal, and passing along 

 the middle of the volution. 



