358 



CRETACEOUS LAMELLIBRANCHIA. 



Distribution. — Hythe Beds of Aylesford, Kent. Mammillatus-bed of Okeford 

 Fitzpaine, Dorset. Recorded by Fitton from the Pema-hed and Ferruginous 

 Sands of Atherfield. Recorded by Topley from the Atherfield Beds of Peasmarsh 

 and Sevenoaks. 



Ostrea cunabula, Seeley, 1861. Text-figs. 141, 142. 



1861. Osteea cunabula, H. G. Seeleij. Ann. Mag. Nat. Hist., ser. 3, vol. vii, p. 



117, pi. v, fig. 1. 

 — — lagena, Seeley. Ibid., p. 117, pi. v, fig. 2. 



Description. — Shell higher than long, either nearly equilateral or slightly or 



Fig. 141. — Ostrea cunabula, Seel. Cambridge Greensand (base of Chalk Marl). Sedgwick Museum. 

 Cambridge. The type of 0. lagena, Seel. Interior and exterior of left valve, x |.. 



moderately inequilateral ; when nearly equilateral the outline is often ovate 

 or oval. 



Left valve with the attached surface either small and near the umbo only, or 

 large — sometimes extending to the greater part of the valve ; in the former case 

 the valve is very convex, in the latter slightly convex or flattened. Umbonal part 

 usually narrow and pointed. Area large, usually higher than long ; sometimes in 

 the plane of the valves and nearly symmetrical ; in other cases curving inwards, 

 outwards or backwards. Surface usually with numerous, rounded, radiating ribs 

 crossed by growth-ridges. Adductor impression deep, near the posterior margin, 

 oval or more or less quadrate. Right valve nearly flat, with growth-lines only. 



Affinities. — 0. cunabula is closely allied to and may be only a variety of 



