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



which oyster attached; attachment to other Ostrea and Inoceramus 

 shells. In some specimens scar area appears secondarily thickened 

 by additional layers of lamellar calcite. 



Ornamentation, left valve. — Variable: Beak and early umbo 

 smooth, or with fine concentric ornamentation. Lower umbo and 

 portion of valve between attachment scar and plicate flanks marked 

 by numerous fine, crowded, overlapping lamellae, fine growth lines, 

 and scattered concentric undulations or moderate size lamellae. All 

 lamellae terminally in contact with succeeding ones. 



Plicae commonly arise abruptly at or slightly below ventral edge of 

 attachment scar (pi. 6, fig. 2). Plicae large, coarse, rounded, broader 

 than interspaces between them, their width greater than their height, 

 subequally developed to irregular (compare pi. 6, fig. 1, with fig, 2), 

 bifurcating irregularly over entire valve; rate of bifurcation greatest 

 dorsally. Secondary plicae rapidly expanding, rarely attaining size 

 of primaries near commissure. Plicae become broader, lower, more 

 rounded, indistinct near ventral and ventrolateral margins of a few 

 valves, rarely disappearing altogether (pi. 6, figs. 2, 4), InterpHcal 

 sulci narrow, sharp to narrowly rounded at base. 



Concentric ornamentation on plicate portion of valve consisting of 

 numerous crowded, subequally to irregularly spaced, fine to medium 

 size, overlapping lamellae, most terminally in contact with succeeding 

 lamellae, and scattered coarse, raised lamellae; latter particularly 

 common near margin. Plicae finely fluted at intersection with major 

 lamellae. Radial and concentric ornamentation of posterior auricle, 

 where developed, more irregular than that of main body of shell ; 

 plicae smaller, more curved and sinuous, more extensively bifur- 

 cating. Concentric elements coarser on auricle. 



Ornamentation, right valve. — Beak smooth ; umbone with scattered 

 fine growth lines and faint concentric undulations. Remainder of 

 valve covered with numerous fine growth lines, scattered flat lamellae 

 of several sizes, and low concentric undulations, irregularly spaced 

 and unequally developed, becoming coarser toward valve margins. 

 Some valves with several coarse lamellae crowded near margins. 



Majority of specimens lack radiating ornamentation. A few right 

 valves have traces of plicae near and on margins. Rare valves 

 exhibit well-developed plicae similar to those of left valve ; plicae 

 arise abruptly near ventral and ventrolateral margins and extend to 

 the edge. Variants of these associated right valves illustrated on 

 plate 6, figures 3, 4, 



