NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 57 



Marginal variants of the Colorado sample, however, show structural 

 features that are not within the range of variation observed for the 

 Texas specimens. The most notable variant, here termed variety A, 

 is a narrow, high form with relatively few plicae and no auricles 

 (pi. 6, figs. 1-6, 19), possibly but not definitely a product of crowded 

 growth conditions. This form is treated separately on subsequent 

 pages. 



A second unusual variant of Colorado L. bellaplicata hellaplicata 

 assemblages is exceptionally long, with abnormally produced anterior 

 and posterior auricles and, in some cases, very broad, rounded, plicae 

 (pi. 6, figs. 11, 15). This form is also represented by a single 

 specimen in the upper Blue Hill Shale, where it is probably a marginal 

 variant of L. bellaplicata novamexicana, gradational into the younger 

 L. hellaplicata bellaplicata. 



Finally, rare specimens of L. bellaplicata bellaplicata from Colorado 

 are nearly smooth, and lack well-developed plicae (pi. 6, figs. 17, 18, 

 21, 24), a condition not attained by Texas representatives of the 

 species. This is most commonly expressed on right valves. 



Analysis of these differences between Texas and Colorado repre- 

 sentatives of L. bellaplicata bellaplicata is difficult, since it is not 

 possible here to separate geographic variance from environmental 

 variance. The shallow-water, high-energy environment indicated by 

 the Codell Sandstone lithology and sedimentary structures is not 

 duplicated in Texas, so there is no basis for comparison of environ- 

 mental influences. I would not consider any of the unique variation 

 in morphology shown by the Colorado sample as being particularly 

 adaptive to the shallower, more turbid, active Codell environment. 

 Modern oysters do not show such structural adaptations in similar 

 niches. This is the only argument that can be presented in favor of the 

 differences being geographic, and thus genetically controlled. Even if 

 this were the case, I would not consider the differences between the 

 Texas and Colorado samples to be of subspecific magnitude, especially 

 since they are shown by a very small percentage of the specimens 

 examined. 



Stratigraphic and geographic occurrence. — In Texas, L. bellaplicata 

 bellaplicata occurs in the upper 50 to 70 feet of the Eagle Ford Shale, 

 being most common in the upper 15 to 25 feet, and forming prominent 

 beds in the upper 2 to 5 feet at various localities. The species has 

 been found in New Mexico at only one locality, in the dark shale of 

 the Benton Subgroup (Mancos), on the Zuni Indian Reservation. 

 In Colorado, the species is restricted to the "Pugnellus Sandstone" 



