442 GEOLOGY OF THE BLACK HILLS. 



Wisconsin, and Minnesota, PI. 8, Fig. 4) that there can be no reasonable 

 doubt as to their being the same variety of this very variable species, to 

 which the name S. nodosus was originally applied. None of the varieties 

 figured by Mr. Meek in the Paleontology of the United States Geological 

 Survey of the Territories approach the features presented by this form very 

 closely or sufficiently near to be considered typical, but this we have 

 deemed best to retain under the original name of the species. It appears 

 to be intermediate between Mr. Meek's varieties brevis and plenus. The 

 general form of the shell is ovate, considerably longer than high, and very 

 ventricose; the inner volutions forming only about half of the length of the 

 shell, and much less than half the bulk. The volutions are closely coiled, 

 leaving but a very slight umbilical opening ; the body volution is some- 

 what regularly arcuate on the periphery and straight on the ventral or 

 umbilical margin of the deflected portion ; the aperture recurving and its 

 margin forming an angle of nearly a hundred degrees with the straight 

 ventral border of the deflected portion, the whole shell sensibly contract- 

 ing above the line of the deflected part to near the aperture, and still more 

 abruptly so j ust at its border. 



The surface of the shell is marked by rather distinct, subangular, sinu- 

 ous costse, which bifurcate so as to form three, four, or five times as many 

 on the broadly-rounded dorsum, which they cross with a slight forward 

 curvature, and also by two rows of rather strong, distinctly elevated nodes 

 or tubercles on each side, situated, one about one-third or nearly one- 

 third of the width of the volution from the margin of the umbilical line, 

 and the other near the border of the rounded dorsum, the latter range 

 being longitudinally compressed, so as to form thin angular nodes. Both 

 ranges of nodes are traceable nearly as far as the volutions are seen. 



The septa are rather complicated and somewhat variable in the differ- 

 ent forms and varieties of the species ; they, however, all have a general 

 resemblance to each other, presenting nearly the same elements in each, 

 and having similar characteristics, which mark them as belonging to the 

 same group or type. These features are shown in the diagram illustrating 

 S. nodosus on Plate 13, Fig. 9. 



