NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 9 



localities and sediment types, representing a broad range of environ- 

 ments. Only in this way could normal variation limits, ecologic 

 control on shell form, and genetic shift be recognized. During the 

 course of this study, I was fortunate to have at my disposal the large 

 and well-documented collections of the U. S. National Museum, and 

 Denver and Washington collections of the U. S. Geological Survey. 

 These were supplemented by my own collections made over a period 

 of 5 years in the Western Interior, and by small lots lent or donated 

 for this study by various individuals. Several hundred measurable 

 specimens, including ontogenetic series, form the basis for species 

 descriptions and observations on stratigraphic distribution, variation, 

 ecology, and evolution. 



Time equivalency of the various fossil beds from one area to another 

 was established on the basis of ammonites, species of Inoceramns, 

 and widespread disconformities. The ammonite and inoceramid 

 species used are widespread, well-established faunal indices occurring 

 in a variety of sediment types (representing various environments) 

 and apparently subject to minimal facies control. 



Simple biometric analysis of all available morphologic features, 

 using graphs, charts, and simple ratios, indicated structures which 

 might be useful in lophid taxonomy, and proved to be sufficient for 

 the recognition of specific differences, ontogenetic development, and 

 evolutionary trends within the Lopha lugubris group. Features that 

 were employed in these aspects of the study are: Maximum adult 

 size ; basic ornament pattern ; normal valve outline ; the distribution of 

 convexity on both valves ; juvenile ornamentation ; extent of valve 

 covered with plicae ; number, size, and bifurcation rates of the plicae 

 at given intervals ; plication density in a given area or distance ; gross 

 characteristics of the concentric ornamentation ; differences between 

 the ornamentation of the auricles and main body of the shell ; relative 

 development of the auricles and auricular sulci ; angle of inclination 

 of the beaks and umbones, and independently of the valve ; curvature 

 and position of the beaks and umbones ; angle of the posterodorsal 

 slope; relative size and development of various parts of the cardinal 

 area; density and extent of the denticles on the dorsolateral valve 

 margins ; relative size and position of the muscle scar ; and in some 

 cases, the relative size, position, and inclination of the attachment 

 scar. To this list might be added comparison of various ratios, such 

 as length to height, length of the auricle to length of the shell, and 

 others. 



Although ontogenetic studies have not been attempted to any extent 



