NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP KAUFFMAN II 



admittedly somewhat idealized. Obviously, it is not possible to 

 evaluate properly all the environmental factors acting on these 

 shells in different areas. The chemical environments of two very 

 similar sandstones may have been entirely different and may have 

 had a profound effect upon the type of shell produced in each area. 

 Analysis of chemical aspects of the paleoenvironment is difficult or 

 impossible in most coarse clastic sediments, and is well beyond the 

 scope of this study. It should be understood that in dealing with 

 environment here, I am dealing in generalities and am cognizant of 

 the limitations this places on the accuracy of my interpretations. 



Gross morphologic similarities in form, ornament pattern, develop- 

 ment of the auricles, the cardinal area, and the muscle scar, in addition 

 to detailed morphologic overlap in marginal variants of consecutive 

 species clearly point out the close relationships of members in the 

 Lopha lugubris group and suggest that they represent a continuous 

 evolutionary sequence, without major break. 



EVOLUTION 



Several of the morphologic characters which are most important in 

 the differentiation of species and subspecies of the Lopha lugubris 

 group demonstrate significant evolutionary trends (figs. 2-9), most 

 of them chronoclinal. These reflect, in part, gradual adaption of the 

 lineage to a slowly changing regional environment characterized by 

 regression and shallowing of the interior seas, increased wave and 

 current action, and increased turbidity. The scope of these changes 

 from one level to another is well beyond the normal ecologic variation 

 caused by similar shallow water conditions in any member of the 

 group during its existence. 



In graphing evolutionary trends in the Lopha lugubris group, 

 structures were compared wherever possible at equivalent ontogenetic 

 stages on the three principal species and subspecies. Comparison of 

 the terminal number of plicae at the margin of adult valves represents 

 such a plot and can be analyzed irrespective of the differences in 

 size range shown by L. lugubris when compared to subspecies of 

 L. bellaplicata. 



In addition to these plots, it seemed desirable to employ another 

 type of comparison in which structures of adult valves from the three 

 forms of Lopha were contrasted at equivalent sizes (i.e., at 20 mm. 

 height) (figs. 6a, 7c). The purpose of this type of plot is twofold: 

 (1) It provides a basis for morphologic comparison of the species 

 and subspecies at equivalent sizes, and a test of their genetic 



