66 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I48 



with margins upturned, forming shallow submarginal trough. High 

 point central to posterocentral. 



Auricles. — Anterior auricle not commonly developed (20 percent 

 of left valves, 5 percent of right valves), dorsoanterior in position, 

 small, flat, subtriangular, terminally rounded, bearing 1 or 2 flattened 

 primary plicae. Anterior auricular sulcus prominent, deeper, broader 

 than adjacent interplical sulci (pi. 7, figs. 10, 11). Posterior auricle 

 small to large, typically of moderate size, flat to slightly convex ; out- 

 line subtriangular, with posterior apex situated centrally on posterior 

 margin (pi. 7, figs. 3, 4, 10, 11). Auricular sulcus very prominent, 

 composed of single broad interplical sulcus, or 2 adjacent interplical 

 sulci with intermediate plica greatly reduced or absent (pi. 7, figs. 10, 

 11). Auricle smaller, less distinct on right valves than on left valves ; 

 rarely absent. 



Beaks and umbos. — Normally slightly opisthogy re, rarely prosogyre, 

 orthogyre, or with strong posterior curvature; slightly projecting, 

 incurved, commonly extending inward beyond plane of commissure 

 on left valve. Beaks small, narrowly to broadly rounded, flat to 

 slightly convex, smooth, situated one-third to slightly less than one- 

 half the length from the anterior margin, commonly obscured on 

 attachment scar. Umbo of left valve highly arched, prominent, 

 plicate. Umbone of right valve flat, smooth to partially plicate. 

 Curvature of umbonal axis gentle, dorsally orthocline to slightly 

 opisthocline, ventrally prosocline. 



Attachment scar. — Small, indistinct, commonly not visible or absent; 

 round to subovate, flat to slightly concave, steeply inclined to plane of 

 commissure ; terminal and central in position, or on posterior slope 

 of early umbo. Attachment to smooth, slightly convex surfaces, pre- 

 dominantly other oysters, smooth clams. Shell moderately thick over 

 attachment area. 



Ornamentation, left valve. — Beak smooth, earliest umbo bearing 

 growth lines, fine concentric lamellae. Plicae originate abruptly on 

 early umbo, at edge of attachment scar (where present), or rarely 

 on middle umbo. Primary plicae typically large, narrow in early 

 stages, broader than high on adult part of shell, becoming broader, 

 more rounded, lower with age, extending to margin on largest valves. 

 Crests of plicae moderately to narrowly rounded, flanks moderately 

 inclined. Plicae bifurcate over entire valve ; bifurcation rate high on 

 umbonal and subumbonal areas, low and irregular over rest of valve. 

 Secondary plicae smaller, narrower, more rapidly expanding than 

 primaries, some attaining size equal to primaries near margin. Rare 



