LA M EL L 1 1) R . I NCIIIA TA . 49 



of the transverse diameter of nearly 45°; margins full, rounded anteriorly, 

 extended on the postero-basal side, and thence following a nearly Btraighl 

 line to the beak. 



Valves similar in general outline. Etighl valve flat or Blightly 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 nf 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 Binus which increases with the 

 age of the shell; margin above the sinus straight, convex or concave; 

 extremity angular. In young specimens the ears are suh-e.pial, 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 ly fine, 

 regular, sharp, elevated, continuous, concentric lines of growth. In the right 



valve the liner radiating lines are conspicuous, while the others are obscure 



and more numerous. The ears do not show the Btronger rays, bui usually 



preserve the liner radiating and concentric lines, and are marked by regular, 

 concentric undulations, which are nol seen on the body of the valves. The 

 thickness of the tot is nut known. 



The hinge-ligament, in mature specimens, lias a width of aboul - nun., 

 and is marked with from four to -even longitudinal, raised lines, or liga- 

 7 



