s 



S 2 



LUGUBRIS 



— I — I — 1 — I — I 1 — I — 1 — I — I — 1 1 — I — r— r 



10 12 14 16 18 20 22 24 26 28 

 HEIGHT OF SMOOTH STAGE (MM.). RIGHT VALVE 



12-1 

 10 

 8 

 6-1 



L. LUGUBRIS 



L. BELLAPLICATA BELLAPLICATA 



LLAPLICATA NOVAMEXICANA 



B 



6 8 10 12 14 16 18 20 22 



HEIGHT OF SMOOTH STAGE (MM.) , LEFT VALVE 



24 26 28 



Fig. 8. — Evolution in the Lopha lugubris group. Increase in the actual height of 

 the smooth umbonal area, dorsal to the abrupt appearance of radiating plicae, 

 in successively younger species and subspecies. A, Right valve. B, Left 

 (attached) valve, on which the trend is best observed. 



20 



