2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



utility of morphologic features not previously considered by many 

 ostreid workers. 



Oysters of the Lopha lugubris group characteristically have a 

 rounded, subovate, or subquadrate outline, asymmetrical prosocline 

 shells with curved, opisthogyrate beaks and umbones (exogyroid in 

 some forms), and coarse, bifurcating, radial plicae originating at the 

 imibone on left valves, but variably developed on right valves. The 

 shell is lamellate and moderately inequivalve, with the right (upper) 

 valve flatter and slightly smaller than the left (lower, attached) 

 valve. The valves are denticulate, with small cardinal areas, and a 

 subcentral, comma-shaped muscle impression. 



The lineage first appears in Western Interior and Gulf Coast 

 sediments during the late Middle Turonian (zone of Collignoniceras 

 hyatti Stanton). The youngest known representatives occur in middle 

 Upper Turonian sediments (zone of Prionocyclus Wyoming ensis 

 elegans Haas). It may range into even younger Carlile strata in the 

 Western Interior, as indicated by scattered reports of small plicate 

 oysters (indeterminate) in uppermost Turonian beds. 



The Lopha lugubris lineage appears to have had a Gulf Coast 

 origin, or center of dispersal, in the United States, and reached its 

 maximum development in abundance and size in these southern 

 waters. Its immediate ancestor is unknown. Possibly, the lineage 

 arose from the European group of L. syphax (Coquand) (Early 

 Cenomanian) or from some Early Cretaceous lophid stock, such as 

 Lopha marcoui (Bose) . There is a distinct gap, however, in our knowl- 

 edge of the evolution of Lopha in sediments of Cenomanian age 

 where representatives of this genus are not common. 



ACKNOWLEDGMENTS 



I am greatly indebted to Drs, Norman F. Sohl and William A. 

 Cobban of the United States Geological Survey, and to Dr. Richard 

 S. Boardman of the United States National Museum, for their excel- 

 lent criticism of the manuscript and many suggestions during the 

 course of the study. Conversations with Dr. H. B. Stenzel of the 

 Shell Development Company were very helpful. Collections were 

 kindly donated to the National Museum by Dr. Bob F. Perkins of the 

 Shell Development Company and loaned by Dr. Donald E. Hattin 

 of the University of Indiana. Specimens that I collected during the 

 period 1958 to 1960 are on loan through the courtesy of the 

 University of Michigan Museum of Paleontology. The drawings 



