450 GEOLOGY OF TUE BLACK HILLS. 



spire, but scarcely increasing in diameter with the increased growth of the 

 shell above the deflected portion, the increased diameter of the tube about 

 equaling the increased diameter of the whorl. 



Surfoce of the shell ornamented by low, subangular, transverse ridges 

 or costae, which pass entirely around the shell, and are separated by wider 

 concave interspaces ; also by two longitudinal rows of obliquely rounded 

 nodes, the upper one of which is placed at or just below the periphery, and the 

 other at about one-eighth of the circumference of the tube below the first, 

 and on the line of the same ridge with it. Transverse costae arranged in 

 sets of three, two of each set coalescing to form the oblique nodes, the third 

 one passing around the tube, without interruption, midway between the 

 two adjacent pairs of nodes. The direction of the costae is strongly back- 

 ward on the upper inner surface of the volution, then recurving and passing 

 obliquely forward in crossing the outer face of the shell from above to the 

 base, and also within the umbilicus; siphuncle small, marginal, situated 

 as far above the upper line of nodes as the distance between the two lines. 



Septa approximate, the extremities of the lobes of one slightly inter- 

 fering with those of the saddles of the next adjacent, but not interlocking ; 

 the space occupied by the convolutions of the lobes and sinuses of a septum 

 is equal to three-fifths of the diameter of the tube at the position of the sep- 

 tum measured ; the ramifications of the lobes are sharply angular and 

 pointed. The siphonal lobe is longer than wide, deeply divided at the 

 extremity into two rather long, compound, digitate processes, and a short, 

 bifid, median process. First lateral lobe very deeply and unequally divided, 

 and the branches widely spreading, and deeply serrate-digitate ; second 

 lateral lobe very much smaller than the first, and less spreading ; antisipho- 

 nal lobe single, longer than wide, deeply digitate on the side, and divided 

 into four small, unequal digitations at the extremity ; sinuses large, spread- 

 ing, and deeply cleft b}' auxiliary or secondary lobes. 



The type specimen of the species is imperfect, but preserves one and 

 one-third volutions ; the volutions are entirely disconnected, and increase 

 rapidly in size, while the umbilicus retains the same diameter, the upper 

 volutions appearing to have been not more closely coiled than those pre- 



