NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 2g 



adjacent layers of gently inclined lamellae commonly with opposed 

 dips. 



Cardinal area of both valves consisting of central triangular resilifer 

 and triangular, subequally developed, lateral cardinal plates. Resilifer 

 of right valve commonly bounded ventrally by a low lip or partial 

 midcardinal fold. Subcardinal cavities commonly absent. Dorso- 

 lateral inner valve margins subequally denticulate ; denticles small, 

 simple, ovoid to elliptical, their long axis perpendicular to commissure. 

 Posterior adductor muscle attachment area posterocentral ; outline 

 comma-shaped ; well defined. 



Remarks. — Many recent workers have included Lopha {= Alec- 

 try onia) in synonymy with Ostrea, or considered it a subgenus or 

 section of Ostrea. Modern representatives of Lopha lack a promyal 

 chamber and are monoecious as are species of Ostrea. Ranson (1942, 

 1948) has demonstrated that the prodissoconch features of Ostrea 

 and Lopha are similar as well. Paleontologists have continued to 

 use the name however, since Lopha is quite distinct from Ostrea 

 in shell sculpture, and the two do not intergrade. Ostrea rarely 

 develops plications, and in cases where they are present, they do not 

 approach those of Lopha in size or development. In addition, species 

 of Ostrea are more equivalve, generally lack curved beaks and umbos, 

 and have a flat commissure. 



Perhaps the most important distinction between Lopha and Ostrea 

 is in the shape of the muscle scar — comma-shaped in Lopha, con- 

 sistently laterally elongate, ovate to kidney-shaped, and larger in 

 Ostrea. This reflects basic differences in the anatomy of the muscle 

 itself. In combination, these distinctions are sufficient to consider 

 Lopha and Ostrea separate genera. Their similarities probably reflect 

 a common ancestry somewhere in the Mesozoic. 



The name Alectryonia has commonly been applied to oysters with 

 the characteristics of Lopha, and the two are objective synonyms, 

 having the same type species. The popularity attained by Alec- 

 tryonia in recent years stems from Stenzel's acceptance of the name in 

 place of Lopha (1947, p. 169, 177) under the rules of the Inter- 

 national Commission of Zoological Nomenclature that existed at the 

 time of his work. Lopha originally appeared in Bolten's Catalogue 

 without description or definition other than "Lopha Der Hahnen- 

 kamm" (Lopha the cock's comb : Stenzel, 1947, p. 177). A list of sev- 

 eral valid species followed, but with no indication as to which was the 

 type. Ostrea crista-galli Linne, now considered the type species of 

 Lopha, headed the list. Subsequently, Alectryonia Fischer de Wald- 



