372 CRETACEOUS LMIELLIBRAXCHIA. 



0. prohosddra, d'Arcliiac, is included b}' Zittel and by G. Midler in 0. vesiadaris, 

 and the latter points out its probable close connection with 0. biatn-ictilata, 

 Lamarck. The type of 0. prohosciilea from the Santonian of Saintes, Dordogne, is 

 not quite satisfactory, but most of the specimens figured by Coquand seem to be 

 inseparable from 0. vesiciilnris. 



The grypheate forms of 0. vpakiilariA have been usually placed in the " genus " 

 Clri/ph.va, and the hippopodutm forms in Odrea; in the case of this species, at any 

 rate, " Gri/phssa " canrwat be accepted as a genus. ^ 



In the Lower Chalk (Plate LV, figs. 8, 9) a small grypheate form" varying 

 only to a small extent in dimensions is common and is not usually associated with 

 larger forms ; the uniformity in size and character of this form at first give the 

 impression that it may be a distinct variety, but comparison with the small forms 

 of 0. vesicularis from the Upper Chalk (figs. 166, 1G7) shows that they cannot be 

 separated ; their small size was probably due to local conditions. Further, it may 

 be noted that in earlier beds (the Upper Greensand) large forms of butli the 

 grypheate and hippopocUum types occur (figs. 143 — 145), and are indistinguishable 

 from those found in the Upper Chalk. Some rather small grj'pheate forms are also 

 found in the Gault and Cambridge Greensand, and also forms with all or the larger 

 part of the left valve attached, but they are not common in the Gault. 



A few specimens of a small oyster have been found in the Lower Greensand of 

 Atherfield and Shanklin, and show the fine radial ribs on the right valve^ ; some 

 of these, and probably all, are examples of 0. rcsinthtri^:, but the left valves are 

 not, in all cases, well preserved.* 



Remarks. — 0. veundaris attains its greatest development in the zones of 

 BeJemnitdla vmcronala and Ostrea lunata in this country, Avhere the grjqjheate 

 forms are abundant and reach a large size. The form with the posterior wing- 

 like projection (figs. 174-179) occurs mainly in the zone of 7)'. mncronatn. 0. /•",>■/- 

 cularis has been observed attached to sponges, corals, echinoids, oysters, Iiioceramus, 

 Spondi/lus, gasteropods. Ammonites, and Belemnitella. The long range in time 

 and the wide geographical distribution of this species have already been noted by 

 Hennig and other authors. 



Tiipps. — 0. vesmdari.i, Lamarck, and 0. ddtoide.a, Lamarck, from the Upper 

 Chalk of Men don. Gri/phsea globo.m, Sowerby, from the zone of Belemnitella 



' Tliis conclusion is in agreement with Peron's views on the " genera " of the Ostreidfe ; see 

 ' Descript. Bracli., etc., Terr. Cret. Tunisie' (1890—111), pp. 107—109. 



2 A similar but rather larger form occurs in the Melbourn Eock and [Aen Ms-niarls. 



' Examples of this are figm-ed in vol. i, pi. v, figs. 4, 6 



■* Ostrea vinjafa, Sowerby, possesses numerous fine radial ribs. The type is the only specimen 

 known ; it was found in the Lower Chalk (probably zone of Holaster gubglohosus) of Sussex. The 

 aflSnities of this form cannot be determined. Dixon, ' Geol. Sussex ' (1850), p. 357, pi. xxvii, fig. 1. 



