36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



lip or partial midcardinal fold (pi. 2, fig. 17). Rarely, entire midcar- 

 dinal fold developed (pi. 2, fig. 14), slightly to moderately convex. 

 Cardinal area of both valves marked by moderately strong, irregularly 

 spaced, crowded, raised horizontal lines transected by finer, more 

 crowded, more evenly spaced, raised vertical lines, forming reticulate 

 pattern. Hinge line short, straight to slightly curved (concave 

 ventrally) , situated ventral to hinge axis. 



Denticles. — Denticles present on inner dorsolateral margins of all 

 valves, rarely extending below upper one-third of margin; small, 

 simple, elongate perpendicular to shell margin, subround to ovate, 

 crowded, distributed along commissure and in shallow trough just 

 inside it ; color whitish, lighter than surrounding shell. 



Inner valve surface. — Commissure undulating, more rarely zigzag. 

 Plicae reflected internally as rounded, low folds and sulci, nearly 

 equal in extent to external trace of plicae, more prominent toward 

 commissure. Fine, irregularly spaced, sinuous, pallial or vascular 

 grooves transgress most of inner shell surface (pi. 1, fig. 17). 



Muscle scar. — Monomyarian, posterior adductor muscle insertion 

 area (scar) subcentral, slightly posterior to midline, comma-shaped 

 to subcrescentic, moderately curved, slightly to moderately concave, 

 prominent. Surface of area marked with faint concentric lines and 

 microlamellae, crowded near ventral and posterior margins, their 

 trace conforming to growth lines. Posterior and ventral part of 

 area bordered by low, raised, lip ; dorsal and dorsoanterior margins 

 overlapped by inner shell layers. 



Shell structure. — ^Valves of medium thickness, variation in thickness 

 moderate. Shell thickest at cardinal area, moderately thick over 

 attachment area, thinning laterally, posteriorly. Thickness of average 

 left valve : at cardinal area 0.9 mm. ; middle of attachment area 0.6 

 mm. ; high point 0.5 mm. ; 2 mm. from ventral margin, 0.5 mm. 

 Periostracum, prismatic layer, hypostracum not observed on sectioned 

 valves. Subnacreous layer forms greatest part of shell, consisting of 

 flat to gently curved calcite lamellae, parallel to one another on free 

 flanks of valve, and curved, slightly inclined plates in distinct sets or 

 layers over attachment scar. Inclination of plates in one layer com- 

 monly opposed to those of adjacent layers (pi. 8, fig. 12). Cardinal 

 area composed of slightly curved, subparallel, thin lamellae of calcite. 



ONTOGENY 



Lopha luguhris is the species least suited for ontogenetic study in 

 this group owing to the size of the attachment scar, which obscures 



