OSTREA. 355 



from the Chalk Marl, and were no doul)t developed for the purpose of fixation in 

 the soft sediment of the sea floor. 



In all cases the right valve starts with a smooth stage, and folds are developed 

 later. The smooth stage corresponds with the attached surface of the left valve, 

 since folding does not begin until the margin of that valve groAvs free from the 

 rock or body to which the shell is fixed. Occasionally, after folding has gone on 

 for some time, a second smooth stage appears on the right valve ; this is due to 

 the left valve becoming again attached to a foreign body (fig. 137). 



Types. — 0. dilaviana, L., from the Senonian of Sweden. 0. j'i'ctiiiatu, Lamarck, 

 is said to have come from near Dreux (Eure-et-Loire). 0. carinata, Lamarck, 

 from Carry (Seine-Inferieure). 0. frons, Parkinson, from France; the locality 

 and horizon are unknown, and the specimen cannot now be found. 0. carinata, 

 Sowerby, from the Upper Greensand of Chute Farm, near Longleat Park, and from 

 the Chalk Marl of Folkestone, are in the British Museum. 0. scrrata, Cuvier and 

 Brongniart, came from Dreux. 0. viacrojitera, Sowerby, from the Lower Greensand 

 of Folkestone, is in the British Museum. 0. retusa, Sowerby, from the Lower 

 Oreensand of Atherfield, cannot be traced. 



Distribution. — Claxby Ironstone (zone of Belemnites lateralis) of South Willing- 

 ham. Tealby Limestone (zone of Belemnites hrunsvicensis) of North AYillingham, 

 Lincolnshire. Ferna-hed of Atherfield. Ferruginous Sands of Atherfield and 

 Shanklin. Recorded by Topley from the Sandgate Beds and Folkestone Beds of 

 Folkestone. LoAver Greensand of Faringdon, Brickhill, Potton and Upware. 



Upper Gault of Folkestone. Cambridge Greensand. Upper Greensand (zone 

 of Schlcenhachia rostrata) of Blackdown, Devizes, "Warminster and Ventnor. 

 L'pper Greensand (zone of Fecten asper) of Devizes. Chalk Marl (zone of Schloni- 

 hachia varians) of Ventnor, Folkestone, Haslingfield, Burwell, etc. Cenomanian 

 Sandstone of Wilmington. Cenomanian (Meyer's Bed 12) of Dunscombe. Zone 

 of Holaster snbglohosus of Cherry Hinton and Burwell. 



OsTKKA LKYirEKir, Lmjmerie [ex Deshayes], 1842. Text-figs. 139, 140. 



1842. OsTREA Leymbeii, [Desbaves, MS.], A. Leijmerie. ili'm. Soc. geol. de 



Fiance, vol. v, pp. 11, 28, pi. xiii, 

 fig. 4. 

 1845. — — E. Forbes. Quart. Joiu-n. Geol. Soc, vol. i, p. 250. 



1843. — — A. Leymerie. Statist, gi'ol. inin. de I'Aube, Atlas, pi. vii, 



fig. 2. 

 1847. — —A. d'Orbigni/. Pal. Frau?. Terr. Cret., vol. iii, p. 704, 



pi. cccclxix, tigs. 1 — 3. 

 1850. — — iVOrhignij. Prodr. de Pal., vol. ii, p. 108. 



1854. — Leymeeiei, J. Morris. Cat. Brit. Foss., ed. 2, p. 173. 



