The IN'ew Specees of Brachiopoba, 349 



Derbya affinis. 

 Plate 5, figs. 10, 11. 

 Derbya affinis, Hall. Palaeontology of N. Y., vol. VIII, pt. 1, p. 349., pi. xi b, 

 figs. 4, 5. 1892. 



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 con- 

 spicuous 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 con- 

 cave 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 direc- 

 tions, being somewhat more abrupt toward the cardinal extremi- 

 ties. This valve also shows a slight tendency to unsymmetrical 

 growth in the umbonal region. 



Surface covered by sharply defined, subequal radiating striae, 

 which increase by implantation. The grooves between these 

 striae are deep, and both striae and grooves are crossed by fine 

 concentric liaes, which on the former produced a series of sharp 

 asperities. Interior, with the exception of the median septum in 

 the pedicle-valve, unknown. 



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



There are many points of similarity in the Orthis KasJcaslciensis^ 

 McChesney, from theKaskaskia limestone, Derbya cymbula and the 

 species under consideration. All have the same general aspect. 

 In 0. Kaskaskiensis the brachial valve is most convex at the 

 umbo, the pedicle-valve generally concave and the hinge line 

 equal to the greatest diameter of the shell ; in Derbya affinis the 

 brachial valve also has its greatest convexity at the umbo, but 

 the hinge line is considerably shorter thaninMoCHESNEv's species, 

 and there is a notable difference in the character of the surface 

 striae; while 'vo. Derbya cymbula the convexity of the brachial 

 valve is greatest at its center, the hinge-line very short and the 

 pedicle- valve concave or depressed only over the pallial region. 



