62 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



REMARKS 



The collections contain no valves with the interior preserved, and 

 the thin, fragile nature of the shell makes it difficult to excavate one 

 successfully. Presumably, the internal structures are the same as 

 those on L. hellaplicata hellaplicata forma typica. Right valves 

 were not found coattached with left valves. Their description is based 

 on numerous smooth valves occurring in the same sediment and con- 

 forming in shape and size to typical left valves. 



It seems advisable to describe separately this marginal variant of 

 L. hellaplicata hellaplicata for two reasons. First, a continuous mor- 

 phologic series cannot be established between the typical form of the 

 species and variety A. The specimens illustrated on plate 6, figures 6 

 and 19, are the only ones that approach the typical form, and these 

 show significant differences. Graphs and charts comparing structures 

 of the two forms invariably show a bimodal distribution with little 

 overlap (figs. 14a-c). There is a possibility, therefore, that the 

 differences between the two forms are not totally environmental, but 

 genetic, and that they were related and coexisting subspecies or 

 species. 



This possibility is further evident considering the apparent environ- 

 ment of deposition of the Codell Sandstone Member. The Codell 

 contains a diverse normal marine, shallow-water invertebrate 

 fauna. The deposit has all the characteristics of a shallow-water, 

 inner sublittoral sand sheet formed under moderate- to high-energy 

 conditions of current and wave action. Variety A and the typical 

 form of the species occur together, represented by numerous well- 

 preserved shells, in the same lenses of fossils. They appear to have 

 lived together and were probably subject to the same environmental 

 influences. L. hellaplicata hellaplicata var. A occurs characteristically 

 in clusters, where the elongate form of the shell appears to be, in part, 

 a product of crowding. But free-growing examples are also known, 

 and these are equally elongate. The form, convexity, and ornamenta- 

 tion are more regular on the free-growing specimens. The differ- 

 ences between the two forms therefore do not appear to be ecologic, 

 or wholly a product of growth habit, indicating that L. hellaplicata 

 hellaplicata var. A may be genetically distinct from the typical form 

 of the species. The small number of specimens available for study, 

 their limited size range, and lack of knowledge concerning the shell 

 interior do not permit verification of these differences, or formal 

 description of a new species or subspecies. Based on modern obser- 

 vations, it seems more likely that a single variable ostreid species 



