NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 55 



subquadrate outline, less oblique shell, gently sloping- posterodorsal 

 margin, more projecting dorsoanterior margin and auricle, larger and 

 better-defined posterior auricle, which is dorsoposterior rather than 

 centroposterior in position, and by its less convex left valve, especially 

 in the umbonal area. In the subspecies hellaplicata, the plicae 

 originate at a later developmental stage, especially on the right valve, 

 and are smaller, narrower, more numerous, more extensively bifur- 

 cating, and differentially developed on the auricles and main body of 

 the valve. The posterior auricular sulcus is narrow and composed of 

 a single, accentuated interplical sulcus. 



The differences between L. hellaplicata hellaplicata and L. luguhris 

 have previously been discussed under "Remarks" for the latter species. 

 L. hellaplicata hellaplicata is easily distinguished from its variety A 

 by being broader, rounder, less erect, auriculate, and in having better 

 defined, much more numerous plicae. 



Among foreign species of Lopha, L. hellaplicata hellaplicata is 

 most closely comparable to Lopha syphax (Coquand), a Lower 

 Cenomanian species which is larger, more coarsely and irregularly 

 ribbed, and has a more pronounced anterior auricle. It further 

 differs from the North American species in being proportionately 

 higher, having a larger posterior auricle, and a more twisted umbone. 

 This form may well be ancestral to the North American group of 

 L. luguhris. 



White (1880, p. 293; 1883, p. 12) gave the name Ostrea 

 (Alectryonia) hlacki (a Lopha) to a variant of L. hellaplicata hella- 

 plicata which he considered distinct on the basis of its greater size, 

 flatter valve, coarser and less numerous plicae, proportionately broader 

 ventral dimension of the valves, and longer, more oblique dorsal 

 margin. He evidently did not recognize the apparent age equivalency 

 of the two forms. Cragin (1893, p. 199) and later writers have 

 established that the two species both came from the upper Eagle Ford 

 Shale of Texas. The present study validates this observation. 



White's syntype lot of L. hlacki consists of 26 specimens, most of 

 them large, displaying late ephebic or gerontic ornament, and obviously 

 worn. Among these latter specimens are the ones White illustrated 

 (1880, pi. 4, figs. 1, 2; 1883, pi. 14, figs, la, b, pi. 17, fig. 5a; 1884, 

 pi. 45, fig. 1, pi. 46, fig. 2), His collection also includes, however, 

 specimens identical to Lopha hellaplicata hellaplicata, and a complete 

 morphologic gradation exists between the two forms in this and other 

 collections from the upper Eagle Ford Shale. 



Differences between the species cited by White break down under 



