LAMELLIBRANCHIATA. 
49 
of the transverse diameter of nearly 45°; margins full, rounded anteriorly, 
extended on the postero-basal side, and thence following a nearly straight 
line to the beak. 
Valves similar in general outline. Right valve flat or slightly concave. 
Left valve moderately convex. 
Hinge-line straight, central, varying from more than one-half the longi¬ 
tudinal diameter in young specimens, to less than one-half in mature 
examples. 
Beak pointed, depressed, directed forward, situated anterior to the middle 
of the hinge-line, and not extending as far as the margin of the hinge. 
Umbonal angle 90°. 
Anterior ear small, triangular, usually less than half the length of the 
posterior one; margin rounded ; extremity obtuse; byssal sinus broad, 
rounded, indenting the body of the shell, and shortening the anterior ear. 
In the right valve the notch is deeper and more angular. Posterior ear 
large, flat, triangular, marked by a distinct sinus which increases with the 
age of the shell; margin above the sinus straight, convex or concave; 
extremity angular. In young specimens the ears are sub-equal, undefined, 
and without sinus. In older shells the anterior ear becomes diminished, or 
appears comparatively smaller; while the posterior ear is increased in size. 
Surface ornamented by from 20 to 25 strong, rounded, continuous rays, 
with broad, flat interspaces which show from three to ten smaller radii; and 
in well-preserved specimens the entire surface of the shell is marked by fine, 
regular, sharp, elevated, continuous, concentric lines of growth. In the right 
valve the finer radiating lines are conspicuous, while the others are obscure 
and more numerous. The ears do not show the stronger rays, but usually 
preserve the finer radiating and concentric lines, and are marked by regular, 
concentric undulations, which are not seen on the body of the valves. The 
thickness of the test is not known. 
The hinge-ligament, in mature specimens, has a width of about 2 mm., 
and is marked with from four to seven longitudinal, raised lines, or liga- 
