NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 49 



slightly concave, bordered ventrally, posteroventrally by low raised 

 lip, overlapped dorsally and anteriorly by inner shell layers ; position 

 subcentral, in posteroventral quadrant of valve on corner nearest 

 center. Surface of area coarsely striated with flat, crowded micro- 

 lamellae, scattered raised ridges, their trace conforming to the growth 

 lines ; rare muscle scars exhibit faint radiating lines. 



Shell structure. — Thickness moderately variable in single valve; 

 greatest in cardinal area and vicinity of beak and umbone, thinning 

 ventrally and laterally. Crest of left valve commonly thinner than 

 shell immediately around it. Shell thickness of average left valve: 

 cardinal area, 4.1 mm. ; crest, 2.9 mm. ; 10 mm. above ventral margin, 

 1.8 mm. Right valve: Cardinal area, 6 mm., center of valve 2.3 

 mm. ; 10 mm. above ventral margin, 1.4 mm. Only subnacreous layer 

 preserved, forming bulk of shell, consisting of subhorizontal to gently 

 curved and inclined plates of calcite arranged en echelon in distinct 

 to roughly defined layers. Inclination of plates in each layer opposed 

 to that in every other, or every third layer adjacent to it (pi. 8, fig. 11). 

 Individual calcite plates thin, moderately short, of irregular thickness, 

 with irregular terminations. Layered arrangement of plates, and 

 opposed inclination of units well defined over most of shell, less 

 distinct in cardinal-umbonal area, where orientation of plates more 

 uniform, not commonly with opposed inclination. 



ONTOGENY 



Numerous well-preserved specimens of Lopha hellaplicata hella- 

 plicata in the collections used for this study retain the morphologic 

 detail of the early ontogenetic stages. The small size, or apparent 

 absence, of the attachment scar on many left valves is of additional 

 help in ontogenetic study since it allows observation of even the 

 earliest stages of the attached valve, an opportunity not afforded by 

 many species of oysters. Graphs and figures depicting the ontogenetic 

 development of L. hellaplicata hellaplicata are presented in figures 

 2-4, 6b, c, 9b, 14-17. Ontogenetic changes were observed in the 

 following structures. 



Concentric sculpture. — Nepionic shell smooth or with faint growth 

 lines; nepionic-neanic houndary marked by a prominent growth 

 line. Fine growth lines characterize early neanic development ; middle 

 and late neanic marked with more crowded growth lines, micro- 

 lamellae, small folds and undulations, becoming progressively coarser. 

 Abrupt appearance of plicae during neanic preceded by a few, closely 

 spaced, moderately coarse lamellae or growth lines, Ephehic stage 



