The Am 
Naturalist. 
from beak to margin, in the center 
of which a shallow furrow is im- 
pressed, Acrotreta Kut 
{cc) Surface rough, ventral valve de- 
pressed-conical, its beak with a 
rounded foramen, and a flattened 
area beneath. Dorsal valve with a 
curved beak arising from two mar- 
ginal projections, Acrothela Linn. 
{a) Shell structure horny, or more or less calcareous {d). 
(d) Shell very small (usually 19 mm. or less) (dd). 
(dd) Form long-oval, surface shining, 
larger valve convex, with a long- 
pointed beak, smaller valve flat, Schinidtia Volb. 
idd) Form oval, both valves convex, 
larger with a short beak, Leptobolus Hall. 
{d) Shell moderate in size, with rounded outlines {e). 
{e) Ventral valve with a marginal sin 
to the beak {ee). 
iee) Marginal sinus relatively nar- 
row, surface calcareous, with 
petal-like markings, beak an- 
imgi 
arly 
Kutorgina Bill. 
{ee) Marginal sinus broad, trian- 
gular or semicircular, beak pos- 
terior, Schizc 
{e) False hinge line straight, nearly 
or quite equaling the breadth of 
the shell. 
W False hinge line, v 
short. Shell nearly _.. , _ 
pressed, smooth, thickened toward 
the hinge. Valves unequal, the 
larger with a shallow groove be- 
neath the beak. The central mus- 
cle scars converere oosteriorlv Obohis Eichwald. 
