72 MISSISSIPPIAN BRACHIOPODA 



wrinkles ; the hinge-line shorter than the greatest width of the shell, the 

 cardinal extremities obtusely angular, the greatest width near the mid- 

 length of the shell. The dimensions of the holotype are : length of pedicle 

 valve 36 mm., length of brachial valve 33.8 mm., greatest width 45 mm., 

 thickness 31.2 mm., length of hinge-line 32.5 mm., height of cardinal area 

 8 mm. 



Pedicle valve moderately deep, the surface flattened in the umbonal 

 region, somewhat compressed towards the cardinal extremities, irregularly 

 depressed or concave towards the lateral and anterior margins; mesial 

 sinus obsolete ; beak not prominent, probably but little or not at all ele- 

 vated above the umbonal surface ; cardinal area of moderate height, slop- 

 ing a little posteriorly from the cardinal line to the apex, its lateral mar- 

 gins sloping in nearly straight lines from the beak to the cardinal extrem- 

 ities, sharply defined in the perfect shell; the delthyrium triangular, 

 higher than wide. Internally the flabellate muscular scar reaches ante- 

 riorly to near the middle of the valve, a median septum is wanting and the 

 dental plates are reduced to low ridges at the sides of the delthyrium. 



Brachial valve strongly convex, much deeper than the pedicle, its great- 

 est convexity a little posterior to the middle, moderately compressed 

 towards the cardinal extremities, the umbo somewhat protuberant beyond 

 the cardinal line; the beak incurved; mesial sinus obsolete. Internally 

 the cardinal process is apparently of moderate size and fills the lower 

 part of the delthyrium of the opposite valve, and the muscular scars 

 occupy a subovate region a little less than one-third the length of the 

 valve. 



Surface of both valves marked by fine radiating costse, from three to 

 four of which occupy the space of one millimeter at the front of the type 

 specimen, upon the partially exfoliated portions the positions of the costas 

 are indicated by lines of papillae. The presence or absence of fine con- 

 centric markings is not indicated upon the type specimen, but both valves 

 are marked by strong and irregular, wrinkle-like concentric markings 

 which give to the entire shell a much distorted and irregular appearance. 



Remarks. This species was established by Hall and Clarke upon a single 

 internal cast from a chert formation, to which some fragments of the shell 

 substance adhere, and it is from this specimen that the above description 

 has been chiefly made. It was originally described as a species of Derbya, 

 although the presence of a median septum in the pedicle valve is not 

 clearly shown as might be expected in such an internal cast. At one 

 point, near the center of the flabellate muscular scar a slight median 

 ridge is suggested, but nothing which can be compared with the strong 

 median septum of other species which have been referred to the genus 

 Derbya. The type specimen is said to have come from the "Keokuk 

 limestone, New Providence, Indiana," but the lithologic character of the 



