BRACHIOPODA. 347 



the most elevated point being a little behind the middle of the valve. There 

 is faint median sinus over the anterior region. 



Surface marked with a few coarse radial ribs, between each two of which 

 are implanted one, two or three much finer ones. These ribs are crossed by 

 a few distinct concentric varices of growth. 



Chester limestone. Crittenden county, Kentucky. 

 This species is readily distinguished by the character of its surface orna- 

 mentation, and though the interior features of the shell are as yet unknown, a 

 very closely allied form from the upper Coal Measures near Kansas City has a 

 well developed median septum in the pedicle-valve, and is hence to be referred 

 to the genus Derbya. 



Derbya Broadheadi, sp. nov. 



PLATE XIA, FIGS. 23, 24. 



Shell with irregularly suboval marginal outline. Hinge-line short, its length 

 not exceeding, and usually less than one-half the greatest diameter of the 

 valves. Cardinal area of the pedicle-valve high, sometimes regularly tri- 

 angular, often distorted or somewhat incurved ; deltidium broad at the base 

 and rapidly tapering with a faint median groove on its surface. Pedicle- 

 valve convex in the umbonal region, irregularly rugose and depressed over 

 the pallial area. Brachial valve very gibbous at the umbo, the greatest con- 

 vexity being behind the center of the valve. From the umbonal region the 

 surface slopes evenly toward the lateral and anterior margins, but is more 

 abruptly depressed toward the cardinal extremities where it forms short sub- 

 alate expansions. The valve is but slightly unsymmetrical and is bilobed 

 by a conspicuous median sinus which takes its origin near the umbo, and 

 widens to the anterior margin. 



Surface covered by fine radiating striae which are of subequal size over 

 the umbonal region, but toward the margin became arranged in fascicles on 

 account of the addition of finer striae as growth advances. Concentric rugae 

 and growth-varices are frequent, especially on the pedicle-valve. 



Upper Coal Measures. Near Kansas City, Missouri. 



