50 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



characterized by scattered coarse lamellae on left valve, some raised 

 above shell surface, and by numerous, closely spaced, very prominent 

 raised lamellae on right valve. Growth lines, microlamellae crowded 

 between major lamellae. Neanic-ephebic boundary marked on many 



/ - ••• ;o / 





V/t- COLORADO (S.S.) 



VALVE LENGTH (MM) 



LEFT VALVE 



-^COLORADO (S.S.) 

 \ 



B 3 4 5 6 7 8 



WIDTH OF MEDIAN PLICA AT 20 MM. HEIGHT (MM) 



LEFT VALVE 



i 5- 



^-^-^ COLORADO (VAR. A) 



lOO" 



4O0JI 



1000 1500 2(300 2500 3000 ' 



area; inscribing rectangle of valve OUTLINE (SQ.MMj 



Fig. 14. — Geographic variation in Lopha bellaplicata bellaplicata; comparison of 

 Texas and Colorado specimens of the typical form of the subspecies, and 

 Colorado representatives of L. bellaplicata bellaplicata variety A. A, Variation 

 in height to length relationship and size range. B, Variation in width of the 

 median plica 20 mm. below the beak. C, Variation in size range, and in number 

 of plicae relative to area of an inscribing rectangle of valve. 



shells by first common occurrence of major lamellae. Lamellae be- 

 come gradually coarser, more elevated, more crowded through 

 ephebic development, probably recording a decrease in growth rate 

 and increase in the length of resting periods with increasing age. 

 Gerontic stage characterized by numerous, coarse, crowded lamellae 

 and coarse growth lines near margin of old shells. 



Radiating sculpture. — Neanic stage marks first abrupt appearance 



