L. LUGUBRIS 



L. BELLAPLICATA BELLAPLICATA 



A 



L. BELLAPLICATA NOVAMEXICANA 



e 8 10 12 14 16 18 20 22 24 26 28 



NUMBER OF DENTICLES IN 5 MM LENGTH 





L. BELLAPLICATA 

 NOVAMEXICANA 



^TA ." ^ • . .; i^" •/ 



• .X \l. BELLAPLICATA BELLAPLICATA 



L. LUGUBRIS 



B 



20 25 30 35 40 45 50 

 LENGTH OF LEFT VALVE (MM) 



55 60 65 70 



Fig. 9. — Evolution in the Lopha lugnbris group. A, Increase in the density of 

 the denticles along the inner lateral and dorsolateral margins in successively 

 younger species. B, Ventral shifting of the beaks along the dorsal margin in 

 successively younger species and subspecies, and decrease in the length of the 

 posterior auricle. Arrow in lower left-hand corner denotes direction of central 

 shifting of beak along dorsal margin as shown by hypothetical regression lines 

 visually fitted to plots of terminal species in lineage. 



21 



