NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 4I 



noted in comparing- suites of specimens from calcarenite, calcareous 

 sandstone, dark shale, and conglomeratic calcarenite facies. The 

 species is amazingly consistent in form for an ostreid. Similarly, 

 there appears to be no recognizable geographic variation between 

 Texas and northern Colorado. There is, however, some suggestion 

 of stratigraphic variation which, when studied with large suites of 

 specimens, may prove to be of subspecific importance. In Colorado 

 and New Mexico, the species ranges through the Juana Lopez 

 Member and its equivalents. In New Mexico, where the Juana Lopez 

 includes a relatively thick series of calcarenites, shales, and calcarous 

 sandstones, specimens of L. lugubris (particularly left valves) from 

 the base of the section appear to be covered to a greater extent with 

 radiating plicae than those from the top, which typically have the 

 plicae limited to the valve margins. The younger specimens also have 

 denser plication. Similar trends were noted at scattered localities in 

 southern Colorado. The proper interpretation of these apparent trends 

 must await larger collections, with better stratigraphic data, than are 

 now available. 



Specimens of Lopha lugubris commonly occur in great numbers in 

 basal Upper Turonian calcarenites of the Juana Lopez Member and 

 its equivalents. The valves are typically unbroken, separated, and 

 oriented with the convex surface upward. They do not occur in beds 

 or "colonies," and very few show any evidence of deformation due to 

 crowding. In many cases, they are the only well-preserved, complete 

 shells in the sediment. 



Of particular interest is the commonly developed reverse curvature 

 of the valves, and the development of exogyroid beaks in either direc- 

 tion. In most species of oysters that develop tightly coiled or exogy- 

 roid beaks and umbones, the direction of coiling is relatively constant 

 and appears to be genetically controlled. Exceptions to the rule are 

 deformed specimens growing in crowded living conditions. There are 

 examples of reverse curvature in many of these species, but it is rare 

 in almost all of them. The unusual coiling behavior of L. lugubris 

 therefore provoked an investigation to see whether it was due to a 

 breakdown in normal genetic control over coiling direction, or whether 

 the species was capable of attachment by either valve. 



Well-developed reverse beak curvature is found in 17.8 percent 

 of all left valves and 23.6 percent of all right valves of L. lugubris. 

 In most of these, the coiling is exogyroid. In all cases of reverse cur- 

 vature the muscle scar retains its position just posterior to the midline 

 and there is no alteration in the structure of the cardinal areas on 



