i8 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



L. BELLAPLICATA 

 BELLAPICATA 



1315 20 25 30 35 'JO 45 50 55 60 65 

 LENGTH OF LEFT VALVE (MM.) 



26 



z 

 526 



124. 



r 



L. BELLAPLICATA 

 BELLAPLICATA 



A A ^ 00 

 I O (^<J>0 O 

 /i^ G A /^QO <9 



/A zao A O A 



A-d^-A CO 1 



LEFT 

 VALVE 



L; BELLAPLICATA 



NOVAMEXICANA 



5 6 e 10 12 14 16 18 20 22 24 

 INITIAL NUMBER OF PLICAE 



Fig. 6. — Evolution in the Lopha 

 lugubris group. A, Variation plot 

 showing relationship between the 

 width of the posterior auricular 

 sulcus 20 mm. below the beak and 

 the length of the left valve, the 

 most variable linear measurement. 

 Note variation within each form 

 and good separation of variation 

 plots, lack of correlation between 

 width and valve length, and de- 

 crease in the relative width of the 

 sulcus at this point in progres- 

 sively younger species and sub- 

 species (the latter defined by differ- 

 ences in the vertical range of the 

 individual variation plots). B, C, 

 Comparison of the bifurcation rates 

 of the radiating plicae on various 

 members of the group, showing an 

 increase in bifurcation of the plicae 

 from L. bellaplicata novamexicana 

 to the younger L. bellaplicata bella- 

 plicata and then an abrupt decrease 

 in bifurcation rate with the evolu- 

 tion of L. lugubris, in which the 

 majority of specimens lack bifurcat- 

 ing plicae. Arrows on right central 

 portion of B and C denote direction 

 of increasing bifurcation rate as 

 illustrated by hypothetical regres- 

 sion lines visually fitted to plots of 

 terminal members of group. 



8 10 12 14 16 IB 20 22 24 

 INITIAL NUMBER OF PLICAE 



