64 University Geological Survey of Kansas. 



Hall's description (in part) : " Shell resupinate, somewhat 

 broadly semielliptical in outline, depressed hemispheric ; car- 

 dinal extremities rounded. Ventral valve flat or slightly con- 

 cave ; area low, extending to the hinge extremities ; foramen 

 forming an equilateral triangle, closed by a pseudo-deltidium. 

 Dorsal valve broadly convex, the greatest convexity a little 

 above the middle, and often equal to one-third the width of the 

 shell. Surface marked by even rounded radiating stride, which 

 increase by bifurcation and interstitial addition, and are crossed 

 by fine concentric striae." Interior of pedicle valve marked by 

 large, deep, semicircular impression, with raised irregular per- 

 iphery, moderately high mesial septum extending to the front of 

 the muscular impression, which is marked by irregular radiat- 

 ing ridges and furrows of variable size. Area of the valve out- 

 side of the impression finely pitted to the edge, where the 

 striations of a new layer of shell form a radiate periphery. 

 Hinge area possessing the usual diverging lines, within which 

 are vertical striae. Entire hinge area marked by horizontal 

 lines or lamellae. 



Range and distribution : Upper Coal Measures to Permian ; 

 Kansas City, Carbondale, Topeka, Cambridge, Cowley county. 



This shell agrees in many respects with the description of D. 

 keokuk and in others with D. robusta. If the previous remarks 

 on surface markings of the valves and the hinge area hold good 

 for the genus, there is little doubt that the two will prove syn- 

 onymous. The fact that our Coal Measures shells have the out- 

 line and also the interior of the pedicle valve almost exactly as 

 D. robusta and, where the shells are not worn, the markings of 

 D. Jceokuk, would seem to indicate that they are the same. 

 Specimens broken from limestone always leave a portion of 

 the shell attached to the stone, which makes the specimens 

 look like those with the markings ascribed to D, robusta. Some 

 of our specimens differ from D. keokuh in that the hinge is 

 hardly as short as ascribed to that species, though some of 

 them approach it quite closely. However, the length of the 

 hinge is not a very constant character in any species of the genus 

 that I have examined. The front and lateral margins expand 



