352 CRETACEOUS LAMELLIBRANCHIA. 



Peron (1908) has already recognised that these " species " cannot be accepted. 

 He states that the detailed analysis of the characters attributed to each of the 

 species and the study of their synonymy shows that the enchainement of these 

 diverse forms is complete, and that usually the difference of names corresponds 

 only to difference of stratigraphical horizon. 



The form found in the Gault, which was named 0. Millctiana by d'Orbigny, is 

 considered by Pictet and Campiche to be identical with 0. carinata of Sowerby 

 from the Upper Greensand and Chalk Marl, which, as already mentioned, is 

 included by d'Orbigny in 0. frons of the Senonian. Pictet and Campiche quote 

 0. Milletiaiia from the Cambridge Greensand, but this is included by Jukes- 

 Browne in 0. frons. 



0. diluviana, Linnaeus, from the Upper Chalk of Sweden, was figured first by 

 Nilsson, and more recently other figures of Swedish specimens have been given by 

 Hennig (1897). D'Orbigny and Coquand recognised that the specimens figured 

 by Nilsson as 0. diluviana could not be separated from 0. frons, and the same 

 view is held by Brauns, Lundgren, and Hennig. The two last-named authors 

 state that Nilsson's figures certainly represent Linnaeus' species, and Brauns also 

 takes them as types of 0. diluviana, but excludes the figures given by d'Orbigny, 

 Coquand, Goldfuss and Geinitz. D'Orbigny, on the other hand, takes Hisinger's 

 figure of a specimen from the Upper Chalk of Sweden as the type of Linnasus' 

 0. diluviana, and excludes from that species the forms figured by Nilsson, which 

 he regards as belonging to 0. frons. The examples of 0. diluviana figured by 

 d'Orbigny, Coquand and Geinitz come from the Cenomanian. In 0. diluviana, as 

 understood by those authors, the shell is oval or rounded in form instead of being- 

 elongate as in the common forms of 0. frons; at first sight it appears to be quite 

 distinct, and being easily recognisable it has been commonly regarded as a separate 

 species, 1 but Lundgren and Hennig have found in the Swedish Chalk a large 

 number of intermediate forms which link together 0. diluviana as interpreted by 

 d'Orbigny and 0. diluviana as figured by Nilsson and Hennig, consequently these 

 two types cannot be any longer regarded as distinct species. Geinitz, although 

 keeping the forms distinct, nevertheless recognises in the Cenomanian of Saxony 

 the existence of intermediate forms, and the same fact is shown by Goldfuss' 

 figures. An oval or rounded form, closely resembling 0. diluviana as understood 

 by d'Orbigny, is found in the Lower Greensand of Faringclon (figs. 115, 118, 

 119), and is certainly inseparable from the common elongate form which has 

 been usually named 0. macroptera ; similar remarks apply to a rounded form found 

 occasionally in the Upper Greensand and Chalk Marl of England (figs. 124, 125). 



1 H. Douville places this in the "genus" Lopha (= Alectryonia), and refers the elongate forms 

 (frons, carinata, etc.) to the " genus " A rctostrea. See 'Bull. Soc. geol.de France,' ser. 4, vol. x 

 (1910), pp. 636, 637. 



