NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 3 



were made by Larry Isham, the photographs by Jack Scott, both of 

 the United States National Museum. 



BIOSTRATIGRAPHY 



Members of the Lopha luguhris group have equivalent stratigraphic 

 distribution in Turonian sediments of Colorado, New Mexico, and 

 Texas (fig. 1). The restricted ranges, broad geographic distribution, 

 and abundance of individual species and subspecies render them use- 

 ful as stratigraphic tools in regional correlation. In some areas they 

 are the best available indices. 



Stratigraphic distribution in Texas. — Lopha bellaplicata novamexi- 

 cana n. subsp. has not yet been found in Texas. Lopha bellaplicata 

 bellaplicata (Shumard) is the oldest known representative of the 

 group on the Gulf Coast, marking a discontinuous faunal zone (zone 

 9 of Adkins and Lozo, 1951, p. 155) at the top of the Eagle Ford 

 Shale (late Middle and early Late Turonian; Cobban and Reeside, 

 1952, chart 10b). It has been reported from the upper Arcadia 

 Park Limestone and Shale Member, the upper South Bosque Marl 

 Member (questionably), and the upper few feet of the "Condensed 

 Zone" of Adkins and Lozo (1951, p. 155). Generally the species 

 occurs only in the upper 25 feet of the Eagle Ford, but locally it has 

 been reported ranging through as much as 70 feet of section. It 

 apparently does not range into the Austin Chalk, although reworked 

 fragments of shells have been found in a thin conglomeratic calcarenite 

 bed ("reworked Eagle Ford" zone) which locally lies between the 

 Austin and Eagle Ford, and which has been assigned by many workers 

 to the former. 



Lopha bellaplicata bellaplicata is known to occur only in upper 

 Middle Turonian sediments in Texas, above the zone of Collignoni- 

 ceras woollgari (Mantell) (early Middle Turonian) and locally below 

 a widespread disconformity which, in the Western Interior, forms the 

 Middle-Upper Turonian boundary. Faunal associates in the U. S. 

 National Museum and U. S. Geological Survey collections from Texas 

 include an undescribed species of Inoceramtis closely related to /. 

 dimidius White (and ancestral to it), Cardium pauperculum Meek, 

 and Cyprimeria? sp. or Tapes sp. cf. T. cyprimeriformis Stanton, all 

 characteristic of the middle Carlile Shale (Blue Hill and Codell Mem- 

 bers) in the southern Western Interior. Lopha blacki (White) 

 morphologically intergrades with L. bellaplicata bellaplicata and 

 comes from approximately equivalent upper Eagle Ford strata. It is 

 here considered a synonym of L. bellaplicata bellaplicata. 



