The American Naturalist. [November, 
{ss) Shell semicircular, often a little 
longer than broad, flat, some- 
times abruptly near the posterior 
^ third, widest at hinge. Both 
valves with an area, that of the 
ventral slightly larger, with a 
small triangular area closed by a 
deltidium. Beak not prominent. 
Hinge plain, Strophoinena Blainv. 
Subgenera. 
Forms with crenulated hinge, Strophodonta Hall. 
Small, more convex forms, vary- 
ing from transverse to nearly 
semicircular, Leptcena Dalm, 
{ss) Shell varying in form and con- 
vexity between Strophomena and 
Orthis. Ventral valve with a 
relatively high area, pseudodel- 
tidium, and prominent, often re- 
curved beak. Dorsal valve with 
a linear area, Streptorhynchus King. 
Subgenus. 
Biconvex forms, with an exceed- 
ingly high area, and coarse 
rounded ribs, separated by 
angular depressions, Meekella W. & St. J. 
(r) Shell biconvex (/). (The convexity of the smaller 
valve sometimes slight). 
{t) See Streptorhynchus and Meekella above. 
(/) Shell depressed, nearly flat, radi- 
ally folded. Valves nearly equal, 
the larger with an area, and 
strongly hooked beak, which is 
closely incurved. No spires, Stricklafidia BilL 
(/) Form quadrate, varying from semicircular. No in- 
ternal spires {tt). 
{tt) Convexity of the smaller valve 
