348 Report of the State Geologist. 



The surface of both valves is covered by fine, elevated, thread- 

 like striae increasing very slowly by intercalation. The edges of 

 these stride bear numerous minute asperities which may be due 

 to the crossing of fine concentric lines. Irregular lines and 

 crinkles of growth are abundant near the margins. 



Upper Coal Measures. Near Kansas City^ Missouri. 



Derbya cymbula. 



Plate 6, figs. 1, 2. 



Derbya cymhida, Hall. Palaeontology of N. Y., vol. VIII, pt. 1, p. 348, pi. 

 xib, figs. 2, 3. 1892. 



Shell large ; marginal outline transversely subelliptical. Hinge- 

 line straight, its length being about two-thirds the greatest 

 diameter of the shell. On the pedicle valve the cardinal area is 

 high, its base being one-third longer than its sides, and it may be 

 somewhat unsymmetrical from distortion. Its surface is finely 

 striated both longitudinally and transversely, and is divided into 

 an outer and inner portion by two lines diverging from the apex 

 and meeting the hinge-line halfway between its extremities and 

 the edges of the deltidium. Deltidium broad at the base, rapidly 

 narrowing for one-third its length, whence tapering more gradually 

 to the apex ; its surface is marked by a well defined median groove 

 for its entire extent. The surface of the valve is elevated in the 

 umbonal region and slopes somewhat irregularly to a low depres- 

 sion over the pallial region and about the margins. The brachial 

 valve is broadly concave at the umbo, but rapidly becomes regu- 

 larly convex, the greatest convexity being in the middle of the 

 valve, whence it slopes almost equally in all directions. There 

 is no evidence of a tendency to irregular growth in this valve. 

 Surface covered with numerous fine, sometimes irregular striae, 

 increasing by implantation. Over the umbonal and pallial 

 regions these striae are of about equal size, but about the margins 

 the tendency to fasciculate arrangement is more apparent. Inte- 

 rior structure, except the existence of a median septum in the 

 pedicle-valve, unknown. 



Upper Coal Measures. J}^ear Kansas City, Missouri. 



