100 University Geological Survey of Kansas. 



leaving a large space beneath ; the shell here is well pitted ; a 

 small tooth and depression are developed on the inner corners 

 of the cardinal area. Shell of the brachial valve thin, muscu- 

 lar marking distinct ; hinge line at beak broadly and shallowly 

 arched, one prominent socket on each side of the arch, for the 

 teeth of the other valve ; two small elevations in the center for at- 

 tachments. The exterior markings of the shell consist of rather 

 large, bifurcating, radiating stride or costse, almost always fascicu- 

 lated, covering the entire shell to the tips of the ears, where 

 they are nearly parallel. There are, in un weathered, unworn 

 specimens, minute pustules arranged in somewhat radiating 

 order, as well as lines of growth visible on the front border. In 

 exfoliated specimens the fasciculation is less distinct. Meas- 

 urements of average specimen: Length, 32 mm. ; breadth, 44 

 mm. ; convexity, 21 mm. 



Range and distribution : Upper Coal Measures ; Kansas 

 City, Lawrence, Lecompton, Topeka, Grand Summit. Com- 

 mon throughout the Upper Coal Measures to the base of the 

 Permian. 



The variation in outline, convexity and length of the hinge 

 line is great in this species. This species has been considered 

 by some as identical with S. striatus ( Martin ) Davidson. Schu- 

 chert has pointed out characters which separate it from that 

 species. 6 There is also a marked difference in the spires of the 

 two, those of S. striatus being long, loose, and acute, while those 

 of S. cameratus are short, compressed, and obtuse, and they en- 

 close a slightly smaller angle. The stride of S. cameratus are 

 nearly always fasciculated, while those of S. striatus are not. 



There is also a marked variation in the interior of the pedicle 

 valve of this shell. In some specimens the muscular scar in 

 this valve is elliptical and extends well back beneath the del- 

 thyrium into the beak, the teeth are not supported by heavy 

 deposits of shell or lamellae, and the cavity formed by the jut- 

 ting of the cardinal area forms a general posterior cavity ex- 

 tending across the shell, only slight ridges being present below 



6. Bull. U. S. Geol. Surv. 87, p. 384: "The latter species [S. striatus], however, is closely and 

 finely reticulated with concentric growth lines, while in 8. cameratus the plications are covered 

 with small pustules, which are arranged in radiating lines." 



