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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



stages (figs. 13b, c). Elements of cardinal area poorly differentiated 

 in neanic stage, becoming well defined at later stages. Extent of 

 denticles relative to height of shell, and density of denticles, greatest 

 during neanic, early ephebic stages, gradually decreasing in later 

 growth stages (fig. 13a). 



Attachment scar. — Rate of size increase relative to total size of 

 shell constant throughout neanic, ephebic stages, demonstrating con- 

 tinued growth of attachment area throughout life. 



Fig. 11. — Ontogeny of Lopha lugnbris (Conrad). Growth line traces at 

 approximately 1.25 mm. intervals on representative left (A) and right (B) 

 valves of the species, showing developmental history of the marginal outline. 

 Drawings X2. A, U.S.N.M. 132156; B, U.S.N.M. 132164. 



REMARKS 



Lopha luguhris (Conrad) is the most distinct member of the group 

 and easily differentiated from related older species. In the Western 

 Interior, it is the terminal member of the lineage known. On the 

 Gulf Coast, however, Lopha panda (Morton) (Campanian, Lower 

 Maestrichtian) possibly represents a stratigraphic extension of the 

 lineage. Forms definitely connecting the two are presently unknown 

 in this country. 



Lopha lugubris may be distinguished from L. bellaplicata bella- 

 plicata (Shumard) and L. bellaplicata novamexicana n. subsp. by its 

 smaller size, greater relative height, much-reduced posterior auricle, 

 general absence of an anterior auricle, subovate to subelliptical mar- 

 ginal outline, shorter, more inclined dorsolateral margins, subcentral 

 and strongly curved to exogyroid beak and umbo, much larger and 

 more gently inclined attachment scar, less convex left valves and 



