8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



overlooked the different stratigraphic ranges of the first two forms. 

 Only mention has been made of the group since that time, and it 

 has not been widely used in biostratigraphic work. The appearance 

 of L. lugubris Conrad on most faunal lists compiled in this century 

 has only limited stratigraphic value in that it is probably used in the 

 concept of Stanton (1893) and indicates an age no more refined than 

 Middle and Late Turonian. 



STUDY OF FOSSIL OSTREIDAE 



The Ostreidae, and in particular Ostrea, Crassostrea, and related 

 genera, are among the most variable of shelled animals. The animal 

 readily adapts to a number of environmental situations without signifi- 

 cant change in morphology of the soft parts. Gross shell form, on 

 the other hand, is greatly affected by the surrounding environment 

 in many species. Long, narrow, straight shells are produced in 

 strong currents ; quiet water favors more rounded, broader shells ; 

 crowding produces elongate, irregular, laterally compressed forms ; 

 the depth and size of the mature shell is related in some groups to the 

 amount of exposure in intertidal areas ; high-energy shallow-water 

 environments produce heavier-shelled, more prominently ribbed forms 

 than do deep, quiet-water niches ; some ostreids form imbricate frills 

 to keep above soft mud and shifting sand bottoms, and so on. Many 

 morphologic features of the shell, therefore, are highly variable within 

 a single species, and must be analyzed with care. This is particularly 

 true in regard to shell outline, convexity, and strength of surface 

 ornamentation. Other structures, less affected by environmental 

 variation (shell structure, prodissoconch, denticles, position and shape 

 of muscle scar, cardinal area), form a more reliable basis for classifi- 

 cation. 



Finally, modern types of Ostreidae (Ostrea, Crassostrea, etc.) are 

 generalized and successful animals, and have been exceptionally con- 

 servative in their evolution. They have undergone little basic change 

 in shell form since they became established as an important group 

 during the Mesozoic. Variants of living species are in many cases 

 indistinguishable from certain Cretaceous forms when only a few 

 shells are compared. Closely related species exhibit considerable 

 morphologic overlap, and the differences between them are commonly 

 subtle. 



In studying the Lopha lugubris group, resolution of the problems 

 inherent in biologic interpretation of the Ostreidae required analysis 

 of large suites of specimens, including growth series, from many 



