lO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 48 



with fossil oysters, distinct and commonly abrupt changes in shell 

 morphology with growth are common, especially in the more ornate 

 groups, and can probably be tied to developmental phases of the 

 animal. In this study the terminology of Hyatt (1894, pp. 349-647) — 

 nepionic (babyhood), neanic (youth), ephebic (adult), and gerontic 

 (senile) — is used to characterize growth periods between major mor- 

 phologic changes in the shell. In oysters, and more specifically in the 

 Lopha higubris group, the nepionic stage is represented by the prodis- 

 soconch, the neanic (spat) stage by part or all of the umbo. The 

 nepionic-neanic transition is marked by development of adult hinge 

 characters, change in shell shape (usually from rounded to dorso- 

 ventrally elongate) , and development of coarser concentric ornamenta- 

 tion. The ephebic stage is characterized by adult ornamentation 

 (the plicae in Lopha), change in shell shape including development 

 of auricles, folds, etc, well-defined denticles and muscle insertion 

 areas, and further differentiation of the cardinal elements. The 

 neanic-ephebic boundary is marked in some lophids by the abrupt 

 appearance of plicae, development of auricular salients in the 

 marginal outline, and differentiation of the resilifer and lateral cardinal 

 plates on the hinge line. The gerontic stage is characterized by 

 flattening and flaring of the ventral and lateral shell margins, deterio- 

 ration of the ornament pattern, decrease in prominence of the radial 

 ornament, crowding and increase in prominence of the concentric 

 ornament, and decrease in prominence of structures related to sexual 

 maturity and reproductive function. 



Graphs and drawings depicting ontogenetic trends in members of 

 the Lopha lugubris group are shown in figures 2-4, 6,9, 11-18. 



Geographic variation and environmental control on shell morphology 

 were determined by comparing a number of "populations" from time 

 equivalent but lithologically distinct sediments in different areas (fig. 

 14). Only L. hellaplicata bellaplicata exhibits significant variation 

 in both respects. Comparison of related forms from a number of 

 different stratigraphic levels, but from similar sediments (roughly 

 representing similar environments) is the most satisfactory method 

 of demonstrating evolutionary change in the lineage. Unfortunately, 

 this method is not wholly applicable to the L. lugubris group, since 

 there is little overlap of exact sediment type between faunal zones. 

 Evolution of this group, therefore, is necessarily determined by com- 

 paring total variation plots of forms from different stratigraphic 

 levels, irrespective of lithology (figs. 2-9). Such a system of separating 

 evolutionary change from geographic or environmental variation is 



