NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP KAUFFMAN 69 



rounded with age through the middle ephehic stage ; becoming lower, 

 more subtle during late ephehic and gerontic stages, especially on 

 left valves. Bifurcation rate highest during neanic, low and irregular 

 at later stages. 



Convexity. — Outward curvature of shell growth moderate to great 

 through neanic and early ephebic development, gradually decreasing 

 in later growth (fig. 3a), becoming moderate to slight. No flattening 

 and flaring noted in late growth stages. 



Marginal outline; auricles. — (Based on fig. 18.) Nepionic shell sub- 

 round, Neanic shell ovate, longer than high ; lateral growth exceeded 



B 



Fig. 18. — Ontogeny of Lopha bellaplicata novamexicana new subspecies. Growth 

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

 right (B) valves of the subspecies, showing developmental history of the 

 marginal outline and the auricles. Drawings Xl- A, U.S.N.M. 132267, 

 B, U.S.N.M. 132265. 



ventral growth during neanic and early ephehic stages. Ephehic stage 

 marked by ventral growth exceeding lateral expansion and the latter 

 becoming irregular. Auricles appear in early ephehic as more rapidly 

 expanding salients of lateral margins, continue to grow at a more 

 accelerated rate throughout life. Gerontic growth similar to that of 

 ephehic stage. 



Curvature of growth axis {midline). — Curvature slight during 

 nepionic, neanic, early ephehic stages, decreasing uniformly through 

 ephehic and gerontic stages, becoming nearly straight (fig. 3c). 



REMARKS 



The distinction between L. hellapliata novamexicana and its most 

 closely comparable relative, L. hellaplicata hellaplicata has been 

 discussed in detail under the "Remarks" section of the latter sub- 



