NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP KAUFFMAN 



19 



8 10 12 14 16 18 2022 24 

 NUMBER OF PRIMARY PLICAE AT 

 FIRST APPEARANCE 



28 



222 

 1^20 

 ?I8 



ol 14- 

 </)|2 



felCH 



UJ o 

 CD 4 



io 



LUGUBRIS 



L. BELLAPLICATA 

 BELLAPLICATA 



L. BELLAPLICATA 

 NOVAMEXICANA 



WIDTH OF MEDIAN PLICA 

 20MM.BEL0WTHEBEAK 



6 8 K) 12 M 16 18 20 22 24 26 



TOTAL PJfMEER OF PLICAE 



AT MARGIN 



Fig. 7. — Evolution in the Lopha lugubris group. A, Increase in total number of 

 radiating plicae developed at the margin of adult shells in progressively 

 younger species of the lineage. B, Increase in the total number of primary 

 plicae first appearing at the margin of the smooth stage on progressively younger 

 species. C, Decrease in the width of the median plica measured 20 mm. below 

 the beak on progressively younger species of the group. All measurements made 

 on left valves. 



