BRACHIOPODA. 349 



thence 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 

 depression over the pallial region and about the mai-gins. The brachial 

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

 lar 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 arrange- 

 ment is more apparent. Interior structure, except the existence of a median 

 septum in the pedicle-valve, unknown. 



Upper Coal Measures. Near Kansas City, Missouri. 



Derby A affinis, sp. no v. 



PLATE XlB, FIGS. 4, 5. 



Shell subsemicircular in outline, somewhat transverse. Hinge-line straight 

 nearly equaling the greatest diameter of the valves. Cardinal area of the 

 pedicle-valve high, its greatest height being about equal to one-third the length 

 of the hinge-line ; divided by diverging lines as in the preceding species and 

 crossed by conspicuous horizontal and fainter vertical striations. This area 

 is often much distorted. Deltidium having a width at the base equal to one- 

 fifth the length of the hinge-line ; it tapers evenly to the apex and bears a 

 median groove on its surface. The umbo is elevated, but the surface of the 

 valve becomes depressed, irregular in growth and concentrically wrinkled, 

 though not concave anteriorly. Brachial valve faintly depressed at the apex, 

 but rapidly becoming convex, the greatest convexity being in the umbonal 

 region, whence the slope is quite regular in all directions, being somewhat 

 more abrupt toward the cardinal extremities. This valve also shows a slight 

 tendency to unsymmetrical growth in the umbonal region. 



