NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN \'J 



4. Variation in the development of the auricles ; a general trend 

 toward reduction in size of the auricles, and elimination of the 

 anterior auricle (fig. 5d). The size of the auricles increases from the 

 subspecies novamexicana to subspecies bellaplicata, and decreases 

 considerably in L. lugubris. 



5. Reduction in the size (width, depth) of the auricular sulci, 

 relative to shell size (figs. 5d, 6a). 



6. Increase in the number of radiating plicae, including an increase 

 in the number of primary plicae at the edge of the smooth stage, 

 increase in number at any given distance below the beak, increase 

 in total number developed at the margin of adult shells, and increase 

 in the plication density (number per unit area) (figs. 2, 7). 



7. Decrease in the average width of the radiating plicae, measured 

 at a uniform distance below the beak in all species (fig. 7c) . 



8. Variation in the amount of bifurcation of primary plicae, with 

 an overall trend toward decrease in bifurcation rate (figs. 6b, c). 

 The bifurcation rate increases slightly from subspecies novamexicana 

 to subspecies bellaplicata, then sharply decreases with the evolution 

 of L. lugubris. 



9. Increase in the absolute height and relative extent of the 

 smooth stage on the early shell, particularly in left valves. This is the 

 distance between the beak and the first abrupt appearance of plicae 



(fig- 8). 



10. Some posterior migration of the beaks and umbos along the 

 dorsal margin, and increase in the amount of curvature of these 

 structures (figs. 3c, Sd, 9b). 



11. Changes in the nature of the cardinal area: general, though 

 not chronoclinal, increase in the size of the resilifer relative to the 

 lateral cardinal areas ; increase in the convexity and prominence of the 

 lateral cardinal folds ; development and accentuation of the marginal 

 cardinal troughs between the lateral folds and the valve margin 

 (figs. 5a-c). 



12. Increase in the density of the denticles on the inner dorso- 

 lateral margins (fig. 9a) . 



Many of these trends are interpreted as adaptations to widespread 

 environmental changes through Middle and Late Turonian time, 

 established in the lineage through natural selection. They are adapta- 

 tions shown by many living and fossil, shallow- water, epi fauna! 

 pelecypods. In the Lopha lugubris lineage they gradually evolved 

 with the steady change from quiet-water, mud-bottom conditions 

 (Blue Hill and lower to middle Mancos Shales), through near-shore. 



