NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 



33 



Table 1. — Summary of measurements on specimens of Lopha lugubris 

 ( Conrad) — continued 



Character Valve 



Width of median plica at 20 mm. H . . ) 



Percentage of plicate valves with 

 bifurcating plicae y- 



(R 



Percentage increase in plicae by 

 bifurcation (measured only on 

 valves with bifurcating plicae) 3 



Percentage of specimens with opis- 



thogyre beaks and umbones 3 



(R 

 Percentage of specimens with or- 



thogyre beaks and umbones 3 



(R 

 Percentage of specimens with proso- 



gyre beaks and umbones 3 



)R 



Length of hinge line (LHL) 3 



I 

 Ratio, LHL:L 3^ 



|r 



Extent of denticles on inner margins 



(in terms of valve height ; HD) 3 



(R 



Ratio, HD:H 3^ 



(R 



Number of denticles in 5 mm. length 

 along dorsolateral margins, just 

 below cardinal area 3 



(R 



Maximum diameter of muscle scar 

 (MDS) jL 



Auricles. — Anterior auricle rarely developed, most common on 

 right valve. Auricle small, rounded, flattened salient of dorsoanterior 

 margin (pi. 2, fig. 5), in some cases separated from shell by shallow, 

 broad anterior auricular sulcus. Posterior auricle present on 59 per- 

 cent of measured specimens ; small to medium size, subtriangular, 

 elongate, flattened, margin rounded, position dorsoposterior (pi. 1, 

 fig. 4). Auricle rarely separated from main body of shell by shallow, 

 broad auricular sulcus. Auricular sulci corresponding to enlarged 

 interplical sulci on plicate valves. 



Beaks, umbones. — Typically opisthogyre, rarely erect, more com- 



