24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



The middle member of the lineage, L. hellapUcata hellaplicata, 

 exhibits more variation in scar size (pi. 3), with individuals that 

 probably lived free during part of their adult life, and others that could 

 well have been attached throughout life. These two forms coexist in 

 the great variety of environments from which the species is known, but 

 there is a notable increase in the relative percentage of individuals 

 with large attachment areas in shallow-water sediments deposited 

 under high-energy wave and current conditions (Codell Sandstone). 

 Scar size thus appears to be largely adaptive and environmentally 

 controlled in this subspecies. 



Small shell size is considered by some ecologists as an adaptation 

 to turbid, near-shore environmental conditions. This may be the 

 case in the L. lugiibris lineage where there is a distinct trend toward 

 reduction in the average adult dimensions with the onset of shallow- 

 water conditions during the Late Turonian. Shallow-water epifaunal 

 elements are able to spend less time feeding than their counterparts 

 in deeper, quieter waters. Long periods during which little feeding 

 takes place are imposed on them by extended times of high turbulence 

 and water agitation. This restriction on food intake has been related 

 to the below average size of many shallow-water epifaunal pelecypod 

 species which develop to normal proportions in quiet, offshore waters. 

 This, of course, is purely an ecologic control on shell form. In the 

 many cases, however, where the smaller of two closely related species 

 or subspecies of epifaunal pelecypods lives closest to the inner sub- 

 littoral zones of constant water agitation and turbidity, small size 

 seems to be a genetic adaptation to the environment. The smaller 

 species, requiring less food, are better able to survive in environments 

 presenting shorter and more irregular feeding opportunities. 



The changes of other structures through time are well defined and 

 easily recognized, but based on my present knowledge of the zoologi- 

 cal characteristics of the group and of the Turonian environment, it is 

 difficult to explain them in terms of adaptive features. 



SHELL MORPHOLOGY 



Standard terminology has been used wherever possible in describing 

 species and subspecies of the Lopha lugubris group (see Newell 

 1937, 1942, Shrock and Twenhofel, 1953, for definitions of standard 

 terms). A few new or rarely used terms are employed here for fea- 

 tures not generally considered in ostreid description. It is desirable to 

 briefly define these. In most cases this is simply accomplished by 

 means of an illustration (fig. 10a, b, c, d), especially in regard to 



