NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 35 



spaced, normally flat, terminally in contact with succeeding ones ; 

 rarely raised. Plicae rugose at intersection with lamellae. 



Ornamentation, right valve. — Ornament of right valve distinct 

 from that of left valve. Beak and early umbonal area smooth ; central, 

 ventral portions of umbo marked with faint growth lines, less com- 

 monly with small, low undulations and microlamellae, rarely with scat- 

 tered coarse lamellae or undulations on ventral umbo. Majority of 

 specimens lack radiating sculpture over rest of valve, but many of these 

 have marginal crenulations near commissure (pi. 2, figs. 1,8). Plicae 

 variously developed on minority of specimens (pi. 2, figs. 2-7), never 

 covering more than half of valve, commonly confined to marginal 5 

 mm. Plicae originate abruptly, on strongly plicate valves at edge of 

 inflated umbonal area (pi. 2, fig. 5) ; plicae prominent, rounded, 

 with steep flanks, becoming coarser with age, rarely bifurcating; 

 secondary plicae smaller, more rapidly expanding than primaries. 

 Plicae rugose at intersection with concentric lamellae ; plicae of right 

 valve more prominent, sharper, than those of left. Interplical sulci 

 deep, narrow, sharp-based, steep-walled. 



Concentric ornament variable, depending on extent of plicae. Low, 

 evenly convex valves lacking plicae covered with faint concentric 

 growth lines, microlamellae, low, broad undulations (pi. 2, fig. 11) 

 except at margin where coarse, crowded lamellae occur. Valves with 

 inflated, smooth umbo and flat flanks (pi. 2, fig. 2) exhibit coarse, 

 crowded, major lamellae, raised growth lines, small prominent undula- 

 tions on flanks ; ornament more prominent near commissure. On 

 plicate valves, prominent lamellae and/or concentric constrictions 

 occur at inner margin of point of origin for plicae (pi. 2, figs. 5, 6). 

 Growth lines and lamellae occur throughout plicate portion of valve. 



Cardinal area. — Composed of subcentral resilifer and/or mid- 

 cardinal fold (right valve), bounded laterally by flat to moderately 

 arched lateral cardinal plates or folds for ligament attachment. Resili- 

 fer of left valve shallow to moderately concave, elongate-triangular 

 with twisted apex, higher than long, slightly flared ventrally, equal to 

 or larger than lateral cardinal plates or folds (pi. 1, fig. 16). Lateral 

 plates, folds subequal, variable in relative size, flat to arched, obscure 

 to prominent, triangular with curved dorsal apices, in some valves 

 separated from lateral margins by narrow lateral cardinal grooves 

 (pi. 2, fig. 17), otherwise merging with margin. Lateral cardinal areas 

 of right valve similar, less commonly arched. 



Central part of cardinal area variable in right valves, ranging from 

 flat midcardinal plate to shallow concave resilifer with raised ventral 



