56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



simple biometric comparison. The syntype lot of L. hlacki contains 

 predominantly large individuals, but all fall within the size limits of 

 L. hellaplicata bellaplicata populations (fig. 16b). The plicae are 

 not less numerous than those of L. bellaplicata bellaplicata (fig. 16c). 

 They appear broader owing to the great amount of wear on certain 

 valves. The valves are neither flatter (fig. 16b), nor broader ventrally 

 than typical examples of L. bellaplicata bellaplicata, and there is total 

 overlap in the inclination of the dorsal margin. Slight differences 

 existing between these forms are probably the product of environ- 

 mental control. Examples of L. blacki are from a relatively coarse 

 argillaceous sandstone, while forms typical of L. bellaplicata bella- 

 plicata occur in finer clastic and mud facies. 



Lopha bellaplicata bellaplicata occurs in a number of different 

 sediments over a broad area and provides a good basis for the study 

 of environmental control on shell form. This opportunity is not 

 presented by other members of the group, which occur in more uni- 

 form lithologies. Large collections of L. bellaplicata bellaplicata were 

 obtained from buff calcarenitic chalk, gray carbonaceous and calcare- 

 ous shale with numerous shell fragments, shaly calcarenite, sandy 

 shale, and in argillaceous to calcareous quartz sandstone (Colorado). 

 A single specimen from New Mexico was found in dark clay shale. 



In collections from Texas, oysters from the calcarenitic marl units 

 are thinner shelled, less convex, less inclined, and have less prominent 

 posterior auricles than those from calcarenitic, sandy, or clay shale, 

 and from thin calcarenites. Specimens from the latter lithology have 

 the largest, thickest shells. These differences are probably related to 

 differences in the energy conditions of the environment. Current and 

 wave action was probably much stronger in the shallow-water environ- 

 ment where the calcarenites were formed than it was in the quieter, 

 presumably deeper marl-forming areas. Thickening of the shell 

 appears to be an adaption to this more active environment, its chief 

 function being to strengthen the valves. 



Specimens from New Mexico, found as gypsum replacements in 

 dark clay shale, are typical of the species. The Colorado sample, 

 however, shows significant environmental, and possibly geographic 

 variation. Specimens from the Codell Sandstone Member ("Pugnellus 

 Sandstone") of south-central Colorado attain a much smaller size than 

 those from Texas and show a somewhat greater variation in marginal 

 outline. The majority of specimens from Colorado, though smaller, 

 are typical of the species and closely comparable morphologically to 

 the Texas forms (compare pi. 3, figs. 1-15, with pi. 6, figs. 7-15). 



