NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN I3 



distinction. (2) At the same time it demonstrates evolutionary 

 trends within the group in absolute measurements. These trends are 

 not as well shown by this type of plot as they might be by comparing 

 structures at the margins of adult shells, where morphologic differences 

 are emphasized, but they are validly illustrated and significant be- 

 cause even this method essentially compares structures at an approxi- 

 mately equivalent stage of development. 



Plots of structures comparing them at a constant height were made 

 within the ephebic part of the shell for each species and subspecies ; 

 thus, only adult structures were compared. Ephebic development 

 starts at about 10 to 15 mm. height on all forms, the difference in 

 adult size between L. lugubris and L. bellaplicata (both subspecies) 

 representing mainly differences in rate and amount of ephebic growth. 

 Since L. bellaplicata bellaplicata and L. bellaplicata novamexicana 

 have nearly equivalent size ranges, comparison of any structure at a 

 given height on both is essentially a comparison made at equivalent 

 ontogenetic stages. Because adult shells of L. lugubris attain a much 

 smaller size, they theoretically should not be ontogenetically compa- 

 rable with subspecies of L. bellaplicata at a given height. Such a com- 

 parison is valid however because the ephebic morphology of this 

 species is strikingly constant. The plicae do not normally bifurcate 

 (fig. 6b and c) and their number is relatively constant anywhere on the 

 adult portion of the shell. In addition, beyond 20 mm. height, the 

 width of the plicae and sulcae remains relatively constant. Since 

 ephebic structures are so uniform in L. lugubris, and values obtained 



Fig. 3. — Evolutionary trends demonstrated by members of the Lopha lugubris 

 group. A, Comparison of cross sections through normal left valves. Beak is to 

 the left. Note general decrease in overall convexity, ventral migration of the 

 high point of the valve (illustrated by vertical lines through cross sections) 

 through time. B, Comparison of cross sections through right valves, beak to the 

 right. Note increase in relative convexity and shift in high point of valve 

 (illustrated by vertical lines through each cross section). C, Comparison of 

 midline traces of typical left valves showing decrease in curvature of beaks, and 

 in overall curvature of shell through time. Beaks dorsal in each case. Speci- 

 mens: A, Top row, left to right, U.S.N.M. 132157, U.M.M.P. 43472, U.S.N.M. 

 132154; middle row, left to right, U.S.N.M. 132307, 132243, 8024b; bottom row, 

 left to right, U.S.N.M. 132276, 132288, 132272. B, Top row, left to right, 

 U.S.N.M. 132164, 132198, 132168; middle rov/, left to right, U.S.N.M. 132306, 

 132305, 132244; bottom row, left to right, U.S.N.M. 132265, 132286, 132287. 

 C, L. lugubris, left to right, U.S.N.M. 132157, 132159, 132156; L. bellaplicata 

 bellaplicata, left to right, U.S.N.M. 132225, 132224, 132222; L. bellaplicata 

 novamexicana, left to right, U.S.N.M. 132275, 132267, 132263. 



