NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP KAUFFMAN 23 



increase in the relative size of the attachment area, the general size 

 and inclination of the area being unusually consistent within species 

 and subspecies. Increase in the size of the attachment area, and 

 decrease in the angle between the area and the commissure both 

 appear to be adaptations to high-energy, near-shore conditions, pro- 

 viding a firmer anchor and lower, hydrodynamically more stable shell 

 in the face of strong current and wave action. There is evidence in 

 the Lopha lugnhris group that these may be genetically controlled 

 features of the shell as well as adaptive variations of individual mem- 

 bers of the lineage. 



Evidence favoring genetic control of inclination and size of the 

 attachment area is found in the end members of the lineage. All 

 observed specimens of L. hellaplicata novamexicana have a very 

 small attachment area. The size of the area relative to the size of the 

 adult shell possibly indicates early detachment of the shell from the 

 substrate and a free-living adult stage. Were this purely an adaptive 

 feature, one would expect it only in quiet-water environments, since 

 a free-living shell of the novamexicana type in normal living position 

 (convex left valve downward) would be highly unstable in the face of 

 strong currents or wave action, and easily overturned and buried. 

 Although sediments containing most L. hellaplicata novamexicana 

 indicate quiet or slightly agitated water conditions, a few specimens 

 occur in coarse-grained, coarse-bedded sandstones of high-energy 

 environments. These maintain the small attachment area. Further, 

 no adaptive counterpart of the subspecies having a large attachment 

 scar is developed in the near-shore facies. Scar size thus appears to 

 be independent of environment in this case. 



Similarly, L. lugubris has an exceptionally large, gently inclined 

 attachment area regardless of associated sediment type and inferred 

 environment. These features indicate attachment throughout life, 

 and are adapted to high-energy conditions. Sediments containing 

 most specimens of L. lugubris reflect such an environment. The 

 species is most common in cross-bedded, ripple-marked calcarenites 

 and calcareous sandstones. A number of examples from thinly 

 laminated shales and fine sandstones and siltstones, however, maintain 

 the large, gently inclined scar, even though it was probably not neces- 

 sary for firm anchorage in the more quiet water environments. 



The change in inferred living habit from partially free to wholly 

 attached, and the consistency in the character of the attachment area 

 on these two forms, regardless of associated environment, suggest 

 genetic control on the size and inclination of the area. 



