442 CrEOLOGY or the black hills. 



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

 (l<tu])t as to tljoir being the same variety of this very variable species, to 

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

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

 Sni'vev 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 

 lo be intermediate between Mr. Meek's varieties hrevis 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 le.ss than half the bulk. The volutions are closely coiled, 

 leaving but a very sligiit 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 

 abiuptly so just at its border. 



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

 ous costae, 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 bv two rows of rather strong:, distinctlv elevated nodes 

 or tubercles on each side, situated, one about one-third or nearl}- 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. )iodosi(s on Plato 1,'J. Fiff. 9. 



