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 tiat 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 b(jdy 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 ;interior ear becomes diminished, or 

 appears comparatively smaller ; while the posterior ear is increased in size. 



Surface ornamented by fi'om 20 to 25 strong, rounded, contiiiuous rays, 

 with broad, flat interspaces whicli 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- 

 7 



