106 University Geological Survey of Kansas. 



Meek's description (in part): " Shell ovoid, being usually 

 widest a little in advance of the middle, and nearly always 

 somewhat longer than wide, moderately convex, becoming 

 rather gibbous with age. Ventral valve usually a little more 

 convex than the other, its greatest convexity being generally 

 behind the middle ; beak prominent, rounded, and distinctly 

 incurved upon that of the other valve ; foramen round, of mod- 

 erate size, and truncating the immediate apex of the beak; 

 mesial sinus absent, or very shallow, in young or compressed 

 individuals, but well denned, and round, flattened or angular 

 in adult gibbous specimens, in which it rapidly increases in 

 size, from near the middle to the front, where it produces a 

 more or less prominent marginal projection, fitting into a cor- 

 responding sinuosity in the margin of the other opposite valve. 

 Dorsal valve moderately convex, the greatest convexity in small 

 or compressed specimens often near the middle or between it 

 and the umbo, but in large, gibbous individuals, with a well- 

 defined, prominent mesial fold, sometimes near the front ; beak 

 rather distinctly incurved under that of the opposite valve. 

 Surface of both valves nearly smooth, or with mere lines of 

 growth, in young shells, but in large or mature specimens with 

 well-defined, imbricating marks of growth on the anterior half ; 

 exfoliated surfaces also show, under a magnifier, obscure traces 

 of radiating striae." The spires are large and generally acutely 

 pointed. Measurements of two specimens: Length, 36 mm., 

 26 mm. ; width, 37 mm., 23 mm. ; convexity, 26 mm., 18 mm. 



Range and distribution : Upper and Lower Coal Measures ; 

 Marmaton station, Bourbon county, Iola, Kansas City, Eudora, 

 Lawrence, Lecompton, Topeka, Manhattan, Grand Summit. 

 Very abundant throughout the Coal Measures of the state. 



Hall has figured this species as having the ascending edges of 

 the spire straight and forming an obtuse angle at the apex. In 

 all the specimens at hand the sides of the spires are concave 

 and the tip of the spire is acute. However, they vary a great 

 deal in this respect. Swallow has described another form, S. 

 caputserpentis, 7 from the Kansas Carboniferous, but I am able 



7. Trans. St. L. Acad. Sci., II, p. 90, 1863. 



