No. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 53 



late neanic or early ephebic stage, corresponding to development of 

 plicae. During ephebic and gerontic development ventral and pos- 

 terior growth exceeded that anteriorly and the auricles continued to 

 expand at a constant, but greater, rate than normal lateral expansion. 



Curvature of growth axis {midline). — (See fig. 3c.) Axis slightly 

 curved in nepionic shell, more moderately curved during neanic 

 and early ephebic development, slightly curved, becoming nearly 

 straight through ephebic and gerontic stages. Inclination of neanic 

 shell possibly opisthocline. Ephebic, gerontic shells prosocline. 



Mtiscle scar. — Scar growth constant through neanic, ephebic stages, 

 at a rate less than overall growth of shell ; growth rate tapers off dur- 

 ing latest ephebic stage (40 to 45 mm.), corresponding to decrease in 

 rate of shell growth, and ceases altogether in gerontic stage (fig. 16a), 

 after animal reaches maximum size. Muscle scar migrates ventrally 

 through neanic, ephebic stages, with increase in shell size. 



Cardinal area. — Elements of cardinal area distinct in early ephebic 

 stage, becoming thicker, broader, better defined at a diminishing rate 

 through life of animal. Length of hinge line gradually increasing 

 through middle ephebic stage at a rate slightly less than overall lateral 

 expansion of valve ; during late ephebic and gerontic stages growth of 

 hinge line continues, but at a rate greater than that of lateral expan- 

 sion of valve (fig. 17c). 



Denticles. — Present in all observed stages, forming throughout life 

 and gradually extending farther down commissure with age. Rate of 

 transgression of denticles down margin (denticle height vs. valve height 

 at various growth stages) uniform and less than rate of ventral shell 

 expansion during neanic through mid-ephebic stages (20-43 mm. 

 height), accelerating and eventually exceeding rate of ventral shell 

 growth in late ephebic and gerontic stages (fig. 19b). Density of 

 denticles decreased at a variable rate throughout growth of shell to 

 gerontic stage (fig. 17a), where it remained stable. 



REMARKS 



Lopha bellaplicata bellaplicata is the best-known species in the 

 L. lugubris group. It is represented by a greater number of individuals, 

 from more localities, and from a broader geographic range than any 

 other member of the group. This is primarily due to its extensive 

 occurrence in the upper Eagle Ford Shale of Texas, and the numerous 

 collections available from that area. 



L. bellaplicata bellaplicata may be distinguished from the mor- 

 phologically similar subspecies novamexicana by its more rounded or 



