NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 47 



Ornamentation, left valve. — Beak smooth or with fine growth Hnes ; 

 first 3-8 mm. of umbo with fine growth Hnes, flat microlamellae, faint 

 concentric undulations, rarely faint radiating undulations (pi. 3, fig. 

 3). Majority of shell marked with coarse radiating plicae transected 

 by prominent concentric lamellae (pi. 3, fig. 15). Plicae arise abruptly 

 on early part of umbo, extend to commissure, increasing in number 

 irregularly through bifurcation, especially on early plicate portion of 

 valve, posterior auricle, and on ventral and lateral margins of large 

 shells. Plicae high, rounded, steep-flanked, broader than angular 

 interplical sulci between them, becoming coarser but lower, more 

 rounded with age, faint or absent near margins of largest valves. 

 Secondary plicae smaller, lower, narrower than primaries but more 

 rapidly expanding, commonly equal to primary plicae in size at valve 

 margin. Posterior auricular plicae smaller, narrower, more divergent, 

 more extensively bifurcating, more curved than those of main body 

 of shell (pi. 3, fig. 11). Plicae most prominent centrally and ventrally 

 on valve; spinose, subnodose, fluted on rare left valves where inter- 

 sected by coarse, raised concentric lamellae (pi. 3, fig, 6; pi. 4, 

 fig. 4). Rare adult shells smooth, or with plicae faintly developed 

 throughout (L. blacki types), expressed mainly as marginal crenula- 

 tions. 



Development of plicae preceded by formation of a few coarse, 

 crowded lamellae on umbone. Concentric sculpture on plicate portion 

 of valve consisting of prominent, moderately to widely spaced, con- 

 centric lamellae separated by numerous, very fine, crowded, irregu- 

 larly spaced, growth lines and microlamellae (pi. 3, fig. 15). Major 

 lamellae terminally in contact with succeeding ones, or raised above 

 valve surface, forming flutes over plicae. 



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

 detail of ornament. Beak smooth or with faint growth lines. Umbo 

 almost totally devoid of plicae; smooth stage higher than that of left 

 valve (pi. 5, fig. 7). Umbo smooth or with very fine, crowded, growth 

 lines and microlamellae ; commonly with narrow zone of crowded, 

 coarse lamellae ventrally, near point of origin of plicae. Plicate 

 portion of valve marked by numerous, raised, closely spaced, major 

 concentric lamellae (pi. 5, fig. 8), becoming crowded near margin of 

 adult valves, much coarser than on left valve. 



Plicae originate abruptly on ventral portion of umbo or below it ; 

 plicae very prominent, high, steep-sided, with sharp to narrowly 

 rounded crests, fluted and spinose at intersection with major concentric 

 lamellae. Interplical sulci narrower than plicae, deep, angular at 



