46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



Rarely, dorsolateral margins moderately rounded or steeply inclined 

 (pi. 5, figs. 7, 10). Left valve typically moderately to highly convex, 

 rarely flattened (pi. 5, figs. 1, 2 typical). High point dorsocentral, 

 commonly on crest of median plica. Slope of anterior, ventral flanks 

 moderate, dorsal slope steep, posterior slope gentle, flattened on 

 auricle. Right valves slightly concave, flat, or gently arched, in rare 

 cases gently and irregularly undulating. High point of right valve on 

 umbone. 



Auricles. — Anterior auricle a flattened dorsoanterior expansion of 

 margin and first few plicae ; auricle small, rounded, semicircular, com- 

 monly absent, in most cases separated from body of shell by an 

 enlarged interplical sulcus, the anterior auricular sulcus (pi. 3, fig. 15). 

 Posterior auricle variable, normally well developed, ranging from 

 indistinct, small, flattened area on dorsoposterior flank (pi. 3, fig. 13) 

 to prominent, flattened, projecting dorsoposterior salient (pi. 4, fig. 2), 

 straight dorsally, narrowly rounded posteriorly, slightly curved to con- 

 cave ventrally, delineated by concave midposterior notch in valve 

 outline. All gradations noted between the two forms. Auricle sepa- 

 rated from body of shell by faint and shallow, to deep and prominent 

 auricular sulcus, an enlarged interplical sulcus (pi. 3, figs. 11, 15), 

 connecting umbo and midposterior notch. 



Beaks, umbos. — Opisthogyre, rarely exogyroid (one volution). 

 Beak of left valve flat, small, bluntly pointed to moderately rounded, 

 commonly obscured by attachment scar. On right valve, posterior 

 curvature of beak, umbo, greater than on left, rarely orthogyre or 

 exogyroid, moderately convex, smooth or with faint growth lines 

 (pi. 5, fig. 7). Left and right umbones typically moderately to highly 

 inflated, rarely flat, right valve with greatest umbonal convexity. 

 Umbonal axes moderately curved, opisthocline medially, acline ven- 

 trally on umbo. Beaks situated anterior to midline, about one-third 

 the length from the anterior margin, slightly elevated above dorsal 

 margin but not projecting. 



Attachment scar. — Predominantly small, commonly minute or 

 apparently missing, rarely large (pi. 3, fig. 17), position middorsal 

 on early part of umbone ; highly variable in shape, normally round to 

 subovate, slightly concave, steeply inclined to plane of commissure. 

 Attachment commonly to small, smooth oyster shells, other Lopha, 

 elongate objects (sticks, etc., but without clasping processes) gastro- 

 pods, and large pelecypods. Shell thin in area of scar, apparently 

 reinforced with secondary calcite layers in some specimens. Shape, 

 size of scar unrelated to that of adult valve, or to plication density. 



