NO. 6 OYSTERS OF TflE LOPHA LUGUBRIS GROUP — KAUFFMAN 67 



valves with sharp, narrow plicae over entire valve (pi. 7, fig. 11), 

 Plicae of posterior auricle smaller, more curved, more irregular, and 

 more extensively bifurcating than those of main body of shell. 



Concentric sculpture over plicate portion of valve consisting of 

 fine, crowded, irregularly spaced growth lines and flat microlamellae 

 dorsally, becoming gradually coarser ventrally. Coarse, raised, major 

 lamellae appear at midvalve (pi. 7, figs. 10, 18), and are scattered 

 singly, or in groups of 2 or 3, over ventral half of valve, intercalated 

 with growth lines and microlamellae. Major lamellae coarsest, most 

 crowded near commissure. Plicae fluted at intersections with major 

 raised lamellae. 



Ornamentation, right valve. — Right valve distinct from left in detail 

 of ornamentation. Beak smooth ; umbo with fine growth lines, micro- 

 lamellae, faint undulations, becoming coarser ventrally. Plicae arise 

 abruptly at edge of umbo, preceded by a few coarse, crowded major 

 lamellae and/or concentric ridges. 75 to 80 percent of valve pli- 

 cate. Plicae coarse, higher than wide initially, becoming broader with 

 age but remaining prominent; crests narrowly rounded to angular, 

 flanks steep ; plicae sparsely bifurcating throughout, generally near 

 margins, much less than on left valve. Interplical sulci narrow, with 

 angular to narrowly rounded bases. Concentric ornament of plicate 

 flanks consisting of numerous, prominent, raised lamellae, moderately 

 and unevenly spaced centrally, crowded ventrally, commonly marking 

 major constrictions or change in slope of shell surface, more promi- 

 nent on right than on left valve. Major lamellae intercalated with nu- 

 merous crowded, growth lines and microlamellae. Plicae spinose, fluted 

 at intersection with major lamellae. Typical right valves illustrated on 

 plate 7, figures 8, 9. 



Cardinal area. — Known from single right and single left valves ; on 

 left valve consisting of subcentral, broad, shallow, triangular resilifer, 

 and equally large, flat, subequal (posterior largest), subtriangular, 

 lateral cardinal plates. Surface of cardinal area marked with closely 

 spaced, moderately strong horizontal lines of various sizes and by 

 fine, crowded, subequally spaced, raised vertical lines. Cardinal area 

 of right valve similar, but with ventral half of resilifer convexly 

 folded, forming a low lip. 



Denticles. — Denticles present on inner dorsolateral margins of 

 both valves ; small, simple, rounded to ovate, crowded, moderately 

 convex, situated on commissure, rarely reflected on thick lateral edges 

 of auricles. 



Commissure. — Commissure at edge of right valve, just within edge 



