372 CRETACEOUS LAMELLIBRANCHIA. 



0. proboscidea, d'Archiac, is included by Zittel and by G. Midler in 0. vesicularis, 

 and the latter points out its probable close connection with 0. biauricul ata , 

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

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

 inseparable from 0. vesicularis. 



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

 Gryphsea, and the Mppopodium forms in Ostrea ; in the case of this species, at any 

 rate, " Gryphsea " cannot be accepted as a genus. 1 



In the Lower Chalk (Plate LV, figs. 8, 9) a small grypheate form 2 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, 167) 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 both the 

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

 from those found in the Upper Chalk. Some rather small grypheate 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 3 ; some 

 of these, and probably all, are examples of 0. vesicularis, but the left valves are 

 not, in all cases, well preserved. 4 



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

 Belemnitella mucronata and Ostrea lunata in this country, where the grypheate 

 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 B. mucronata. 0. vesi- 

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

 Spondylus, 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. 



Tapes. — 0. vesicularis, Lamarck, and 0. deltoidea, Lamarck, from the Upper 

 Chalk of Meudon. Gryphsea globosa, Sowerby, from the zone of Belemnitella 



1 This conclusion is in agreement with Peron's views on the "genera" of the Ostreidse; see 

 'Descript. Brach., etc., Terr. Cret. Tunisie' (1890—91), pp. 107—109. 



2 A similar but rather larger form occurs in the Melbourn Rock and jjJenus-iw&vls. 



3 Examples of this are figured in vol. i, pi. v, figs. 4, 5 



4 Ostrea virgata, Sowerby, possesses numerous fine radial ribs. The tjpe is the only specimen 

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

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



