34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



monly prosogyre, predominantly exogyroid on left valves, attaining 

 three-quarters to one and one-half volutions (pi. 1, fig. 5, text fig. 3c) 

 on both prosogyre and opisthogyre shells ; strongly curved where not 

 exogyroid, Beaks, umbos moderately curved posteriorly on most 

 right valves. Beak pointed (right valve) to narrowly rounded (left 

 valve), situated centrodorsally or just posterior to midline. Prodis- 

 soconch rarely preserved, smooth, flat, separated from umbo by single 

 prominent growth line or concentric constriction. Beak, umbo flat 

 to concave on left valve, deformed on attachment scar; slightly to 

 moderately inflated on right valve, in some cases separated from rest 

 of valve by prominent constriction and/or abrupt change in ornament 

 (pi. 2, figs. 2, 5, 6). 



Attachment area. — Situated on dorsoposterior flank, large on most 

 left valves (pi. 1, figs. 1-14), equal to one-half area of shell, flat to 

 concave, gently inclined to plane of commissure (average angle 24°), 

 outline rounded to subovate, similar to ultimate shape of valve. 

 Ornament of attachment area reflects surface to which attached, nor- 

 mally Ostrea and Inoceramus shells; area marked with concentric 

 lines and folds. No clasping processes developed. Attachment area 

 commonly obscures beak, umbo. 



Ornamentation, left valve. — Beak smooth; early part of umbo 

 smooth, central and ventral portions covered with fine growth lines, 

 small concentric undulations. Free surfaces of most valves covered 

 with coarse, simple (primary), radiating plicae (pi. 1, figs. 1, 2, 4-6), 

 originating abruptly at or near margin of attachment area and extend- 

 ing to commissure, rarely bifurcating; rare valves smooth, with plicae 

 reflected as marginal crenulations, or with plicae greatly reduced in 

 prominence (pi. 1, figs. 3, 11-14). All gradations known between 

 smooth and fully plicate forms. Primary plicae straight to slightly 

 curved, slightly broader than high, crests rounded, flanks steep, more 

 prominent centrally than laterally, becoming higher, more angular, 

 broader, more prominent with age. Secondary plicae smaller, lower 

 than primaries, more rapidly expanding in size (pi. 1, fig. 4). 

 Interplical sulci deep, steep-walled, narrower than plicae, angular to 

 narrowly rounded at base. Commissure strongly undulating, rarely 

 zigzag, at intersection with plicae. 



Concentric ornament between edge of attachment area and plicate 

 portion of valve with faint growth lines, microlamellae, more rarely 

 with crowded, coarse, flat, major overlapping lamellae. Plicate portion 

 of valve characterized by moderately spaced to crowded overlapping 

 lamellae separated by very fine growth lines. Lamellae irregularly 



