370 CRETACEOUS LAMELLIBRANCHIA. 



either nearly flat (fig. 151) or grows upward near the margin only (fig. 145), 

 and the right valve is either slightly convex throughout (figs. 147, 150) or with 

 a narrow marginal part growing upwards (fig. 155). Although this type differs 

 greatly in appearance from the grypheate form (fig. 182), yet every stage between 

 the two extremes can be seen since there is a complete gradation from forms in 

 which the entire surface is attached to forms in which only a very small part of 

 the umbo is fixed. But whatever the form of the shell the part, if any, which 

 grows free shows similar characters. This view of the relationship of 0. hippo- 

 podium and 0. vesicular is has already been maintained by Peron (1890) and by 

 Muller (1898). 



In the Chalk of this country numerous examples of the left valve of an oyster 

 attached completely to flat or convex objects are found and have been commonly 

 referred to 0. Normaniana, d'Orbigny 1 (fig. 140). The right valve is nearly always 

 missing. The marginal part of the left valve thins away rapidly to a sharp edge 

 and this part usually shows a porous structure, whilst on the inner part of the 

 valve a few distinct concentric ridges are seen ; some of these specimens are young- 

 individuals and many are probably immature ; the concentric ridges probably 

 indicate the beginning of an upward growth of the margin which was soon dis- 

 continued and the attached growth resumed. Similar concentric ridges are present 

 in some undoubted examples of 0. vesicularis (figs. 148, 149), and the identity of 

 this form with 0. vesicularis is proved by the occasional preservation in its original 

 position of the right valve (Plate LV, fig. 4; text-fig. 147) showing the charac- 

 teristic radial ornamentation of 0. vesicularis. A left valve of this form is figured 

 by Geinitz 2 as 0. hippopodium. 



Avicula filata, Etheridge, from the Totternhoe Stone (zone of Holaster sub- 

 globosus) of Burwell, is founded on a right valve of a small specimen of 0. vesicu- 

 laris, and shows clearly the fine radial ribs. It agrees with small specimens 

 attached throughout or with only the margin free, found in the Upper and Lower 

 Chalk as well as in the Gault 3 (Plate LV, figs. 4, 5, 7; text-fig. 147). 



0. clavata, Nilsson, is a form in which the posterior part is more produced than 

 usual owing to the position of the attached surface. 



The hippopodium form of 0. vesicularis found in the Cenomanian was named 

 0. Lesueuri by d'Orbigny on account of its lower horizon. But Hennig, who has 

 studied a series of Cenomanian specimens, sees no reason for separating it from 

 the Senonian examples, and Muller (1898) likewise includes it in 0. vesicularis. 



1 « Pal. Fraii^ Terr. Cret.,' vol. iii (1847), p. 746, pi. cccclxxxviii, figs. 1—3. 



2 ' Palseontographica,' vol. xx, pt. 2, pi. viii, fig. 6. 



3 Anomia subradiata, Reuss, is probably a small example of 0. vesicularis similar to Avicula 

 filata of Etheridge. Reuss, 'Die Verstein. der bohm. Kreideformat.,' pt. 2 (1846), p. 45, pi. xxxi, 

 fig. 19. 



