NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 5 1 



of plicae, anywhere from earliest to late neanic in left valves, at or 

 near beginning of ephehic development in right valves. Plicae be- 

 come gradually coarser with age to late ephehic stage on left valves 

 (about 40 mm. height) ; beyond this they become fainter, broad, low 

 folds, many of which disappear with continued deterioration of 

 ornamentation during gerontic development. Plicae of right valve 

 become coarser with age through at least early gerontic stage. Plicae 

 bifurcate throughout development; bifurcation greatest in late neanic 

 or early ephebic stage, again at late ephehic development, the former 

 possibly marking a period of accelerated growth. 



Fig. 15. — Ontogeny of Lopha bellaplicata bellaplicata (Shumard). Growth line 

 traces at approximately 2.5 mm. intervals on representative left (A) and right 

 (B) valves of the species. Traces show developmental history of the valve 

 outline and the auricles. Drawings XL A, U.S.N.M. 132225; B, U.S.N.M. 

 132239. 



Convexity. — Outward curvature of valves (convexity) slight in 

 nepionic shell, on the left valve (fig. 3a) becoming moderate to great 

 during neanic and early ephehic development, decreasing through 

 late ephehic stage. Relatively abrupt flattening, flaring of flanks 

 characterizes gerontic development. On right valves (fig. 3b), 

 moderate outward curvature characterizes neanic, earliest ephehic 

 stages ; gradual to abrupt flattening occurs during most of ephehic 

 stage. Upturning of valve margin and flaring are common gerontic 

 characters. 



Valve outline and auricles. — (Based on fig. 15.) Nepionic shell 

 subround. Neanic shell ovate, with ventral growth exceeding lateral 

 growth. Auricles appear as faint salients of dorsolateral margins in 



