68 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



of left valve ; flat dorsolaterally, undulating at intersection with major 

 plicae elsewhere. Internal expression of plicae variable, typically 

 developed as low, rounded folds and sulci confined to valve margins, 

 less commonly extending to center of shell, becoming more subtle 

 inward. Thick-shelled specimens commonly with lamellate lateral 

 edges of shell flattened in plane of commissure (pi. 7, fig. 5) . 



Muscle scar. — Monomyarian ; posterior adductor scar large, situated 

 centroposteriorly, or in upper part of posteroventral quadrant of valve 

 (pi. 7, fig. 5) ; comma-shaped, arcuate, slightly concave, well defined, 

 overlapped dorsally, anteriorly by inner shell layers, bordered ventrally 

 by a low, raised lip. Surface striated, covered with fine, crowded, 

 microlamellae and raised lines, irregularly spaced, conforming to 

 trace of growth lines. 



Shell structure. — Periostracum, prismatic layer, and hypostracum 

 not observed. Subnacreous layer forms bulk of shell, consisting of 

 several layers of slightly curved, inclined calcite plates arranged en 

 echelon within each layer. Inclination of plates in each layer may be 

 opposed to that of adjacent layers. Individual plates and layers 

 flatter, more conformable around cardinal area and at ventral edge. 

 Nacreous layer thin, composed of flat calcite lamellae. Shell thick; 

 thickness of a typical left valve (height 38 mm., length 34 mm.) 

 through the hinge area, 4.5 mm. ; thickness at crest, 3.4 mm. ; thickness 

 5 mm. above ventral margin, 2.3 mm. 



ONTOGENY 



A few significant ontogenetic trends regarding the external features 

 of the shell are worthy of note. Internal structures of the valves are 

 virtually unknown in this respect. 



Concentric sculpture. — Nepionic stage smooth. Neanic stage with 

 fine growth lines, microlamellae initially, becoming coarser toward 

 late neanic. Early to middle neanic in left valve marked by a few, 

 coarse, crowded growth lines, lamellae, and ridges just before abrupt 

 development of plicae. Ephebic stage marked by development of 

 scattered (left valve) to moderately spaced (right valve) major con- 

 centric lamellae in addition to fine ornament, possibly marking growth 

 rests, and becoming somewhat more crowded in late ephebic as 

 growth slowed. Gerontic stage marked by crowded, coarse lamellae 

 and growth lines near margin. 



Radiating sculpture. — Plicae abruptly appear in early, rarely middle 

 neanic stage on left valves, at the beginning of ephebic development 

 on right valves, becoming coarser, more prominent, broader, more 



