470 MISSISSIPPIAN BRACHIOPODA 



1897. Athyris densa Hall, 14th Rep. N. Y. State GeoL, p. 358, pi. 9, figs. 3-9. 

 1906. Centronella ? f crassicardinalis Beede, 30th Ann. Rep. Geol. Surv. 



Ind., p. 1308, pi. 22, figs. 50-52. 

 1906. Athyris densa Beede, 30th Ann. R.ep. Geol. Surv. Ind., p. 1320, pi. 19, 



figs. 2-2e. 



Description. Shell below medium size, wider than long, the valves 

 much thickened posteriorly, unequally convex, the greatest width near 

 the mid-length of the shell, the hinge-line arcuate, shorter than the 

 greatest width, the cardinal extremities rounded. The dimensions of a 

 complete specimen, one of the co-types of the species, are: length 18 

 mm., width 22 mm., thickness 11.5 mm. 



Pedicle valve shallow, depressed in its posterior half, curving towards 

 the opposite valve anteriorly, the surface nearly straight, gently convex 

 or sometimes a little concave along the transverse line of the valve be- 

 tween the points of greatest lateral extent, except near the lateral 

 margins where it is abruptly bent towards the opposite valve and is even 

 inflected in the larger examples ; the beak not incurved, contiguous with 

 the umbo of the opposite valve, pierced by a small sub-semicircular 

 foramen which is joined broadly with the apex of the delthyrium; the 

 false cardinal area sharply defined along its ventral margin or curving 

 very abruptly into the lateral slopes of the valve, rather broad and nearly 

 flat, or slightly concave from the cardinal to the ventral margins, the 

 two portions of the area on opposite sides of the delthyrium not lying 

 in a plane, if produced to the center of the delthyrium they would meet 

 in an obtuse angle, laterally the area continues on each side with the 

 curvature of the valve nearly or quite to the greatest transverse diameter 

 of the valve; delthyrium broadly triangular, wider than high, nearly 

 filled by the beak and cardinal processes of the opposite valve when the 

 valves are in articulation, the apex connected with the foramen; lateral 

 slopes depressed, nearly straight transversely, but convex antero-poster- 

 iorly; mesial sinus originating near or at some distance anterior to the 

 beak, shallow, usually narrow but sometimes broad and ill-defined, pro- 

 duced anteriorly in adult individuals. 



Brachial valve more strongly convex than the pedicle, the greatest 

 convexity near the middle, the surface sloping with a convex curvature 

 from the median line to the lateral margins, the curvature somewhat 

 more abrupt towards the cardinal and anterior margins; the beak in- 

 curved beneath that of the opposite valve; mesial fold ill-defined later- 

 ally, passing without demarkation into the lateral slopes, flattene'd on top 

 or sometimes with a median longitudinal depression tow r ards the front. 



Surface of both valves marked by fine, sublamellose, concentric lines 

 of growth, which are crowded at intervals, especially towards the front, 

 to form much stronger growth lines. 



