FOSSILS OF THE ST. LOUIS GROUP. 515 





 and a half, increasing rapidly in size, and a little wider 



transversely than their dorso -ventral diameter; moderately 

 embracing, with a subquadrangular section, the angles 

 being ronuded, and the lateral and onter or ventral surfaces 

 more or less flattened. Septa moderately concave, distant 

 about one-fourth the transverse diameter of the volutions 

 at the point of measurement, and showing a slight back- 

 ward curve in crossing the sides and periphery; aperture 

 large, and, as near as can be determined from the speci- 

 mens, with a subquadrangular or subcircular outline, more 

 or le.ss sinuous on the inner side for the reception of the 

 small inner turn; siphuncle small. Surface smooth, or 

 only showing small lines of growth. 



Greatest diameter of a small specimen, with body cham- 

 ber broken away, 1.70 inch; transverse diameter of same, 

 about 1.25 inch. 



As in other species of this group, the small siphimcle is so very nearly 

 iii contact with the outer side that in casts where the shell is removed 

 it often gives the appearance of a very narrow, deep lobe along the 

 middle of that side. It is easy to see, however, that this appearance 

 is merely produced by the breaking away of a thin part of the matrix 

 between the siphimcle and the outer shell. Xone of our specimens are 

 in a condition to show the margins of the lip, but some of them show 

 very clearly the commencement of the protuberance or pinching up of 

 the margin on each side near the umbilicus, evidently terminating at 

 the aperture in the usual spout-like auricles. The lines of growth 

 also show the same by the flexures on each side. 



Specifically tbis shell is probably most nearly allied to our jV~. (tioleno- 

 cheilm) Leidyi, from the Keokuk division of the Lower Carboniferous, 

 though it diners in having more rapidly expanding and subquadrangu- 

 lar whorls, which are also slightly embracing at the aperture, instead 

 of being merely in contact. Its volutions, however, are much less rap- 

 idly expanding than in our X. (Sol.) capax, or in N. Springeri, of White 

 and St. John, as well as different from both in their subquadrangular 

 form. 



Locality and position. St. Louis division of the Lower Carboniferous 

 scries, Greencastle, Indiana, and near Waterloo, Monroe county, HL 



70 



