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



the nepionic and neanic development of many left valves, the lack of 

 immature shells in the collections, and the limited size range of the 

 species. Ontogenetic trends can be studied in the following structures, 

 which are graphed or illustrated in figures 2-4, 6b, c, 9, 11-13. 



Concentric ornament. — Nepionic shell smooth, transition to neanic 

 marked by single growth line or small constriction. Neanic stage with 

 fine growth lines, becoming coarser, more crowded, mixed with micro- 

 lamellae ventrally. Neanic-ephebic boundary poorly defined. Ephe- 

 hic stage with coarse growth lines, microlamellae and scattered macro- 

 lamellae, faint undulations, on left valve. Right valve with faint 

 growth lines during early ephebic stage (ventral umbo), abrupt con- 

 striction of shell at midephebic stage, marking cessation of major 

 growth in body size, prominent lamellae, growth lines in late ephebic. 

 Gerontic stage characterized by crowded, coarse major lamellae, small 

 undulations near margin, marking great reduction in rate of shell 

 growth, sharp change in concentric ornament marks ephebic — gerontic 

 boundary. 



Radiating ornament. — Plicae appear abruptly in middle to late 

 ephebic (most left, some right valves), rarely earlier, marking point 

 where shell begins to grow free of substrate ; plicae becoming higher, 

 broader, sharper, and rarely bifurcating through late ephebic, gerontic 

 stages. 



Marginal outline. — (See fig. 11.) Nepionic: Prodissoconch sub- 

 round. Neanic: Ventral growth exceeds lateral, outline vertically 

 ovate, slight expansion of posterior flank. Ephebic: Ventral and 

 posterior growth exceed that anteriorly and are nearly equal. Auricles 

 form in early ephebic stage, expand with age. Relationship between 

 expansion rate of auricle and overall lateral ephebic growth constant 

 (fig. 12a). Gerontic: Outline remains essentially the same, slight 

 flaring of ventral, ventrolateral margins. 



Curvature of midline axis. — Nepionic: Slight, poorly known. 

 Neanic: Moderate to great (fig. 3c), gradually decreasing in late 

 neanic stage and through ephebic and gerontic stages, becoming slight. 



Convexity. — On right valve (fig. 3b), gradual increase in degree of 

 outward curvature from nepionic through early neanic, greatest in 

 late neanic, early ephebic, gradually to abruptly decreasing through 

 late ephebic. Gerontic stage marked by abrupt flattening, upturning 

 of margin. 



Internal structures. — Rate of expansion of muscle scar, hinge line, 

 constant, and less than rate of shell growth during neanic and early 

 ephebic stages, tapered off gradually during late ephebic, and gerontic 



