ATHYRIS 471 



Remarks. This species may be easily recognized by the transversely 

 flattened surface of the pedicle valve, and by the conspicuous false 

 cardinal area of the same valve. It was originally described by "Whitfield 

 as Centronella crassicardinalis, but is clearly not a member of that genus. 

 \Vhitfield 's specific name is preoccupied in the genus Athyris by A. 

 crassicardinalis White, consequently the name A. densa H. and C., must 

 be retained for the species. Whitfield 's description was based upon a 

 single, small pedicle valve which was proportionally somewhat more 

 elongate than usual, but is clearly the same as Hall and Clarke's 

 species. 



Horizon. Salem limestone. 



ATHYRIS CRASSICARDINALIS White 

 Plate LXXVII, Figs. 1-20 



1860. Athyris crassicardinalis White, Jour. Boston Soc. Nat. Hist., vol. 7, 



p. 229. 

 1899. Cliothyris crassicardinalis Girty, Mon. U. S. Geol. Survey, vol. 32, 



p. 567, pi. 71, fig. 8a, 

 1901. Athyris crassicardinalis Weller, Trans. St. Louis Acad. Sci., vol. 11, 



p. 186, pi. 16, figs. 18-24. 



Description. Shell small, biconvex, suboval or subcircular in outline, 

 the width equal to or greater than the length, the greatest width near the 

 mid-length of the shell, the hinge-line shorter than the greatest width, the 

 cardinal extremities rounded. The dimensions of two pedicle valves 

 are: length 9 mm. and 10.3 mm., width 10 mm. and 10.2 mm., convexity 

 4.5 mm and 4 mm. The dimensions of a brachial valve are: length 8 

 mm., width 9 mm., convexity 3 mm. 



Pedicle valve strongly convex, the greatest convexity near or posterior 

 to the middle, most elevated along the median line, the surface curving 

 abruptly from the umbonal region towards the cardinal margin and 

 sloping laterally from the median line with a gently convex curvature ; 

 median sinus obsolete, the median portion of the valve slightly flattened 

 in an ill-defined, narrow band from which the surface slopes laterally in 

 each direction; beak small, pointed, moderately incurved; cardinal area 

 obsolete; delthyrium large, open, broadly triangular. Internally the 

 hinge-teeth are large and strong, they are nearly horizontal in position 

 and are directed medially, growing out from the inner surface of the 

 valve as shelf-like processes which are concave externally; the muscular 

 scars are large but ill-defined, sometimes divided longitudinally by a 

 slightly raised rib. 



Brachial valve much less convex than the pedicle valve, arched from 

 the posterior to the anterior margins, depressed-convex transversely; 

 mesial fold obsolete, the median portion of the valve slightly flattened 

 along a narrow, ill-defined region from which the surface slopes laterally ; 



