A small oyster, called _Ostrea vesicularis_, is a characteristic shell of the chalk; one valve is convex, the other flat; it is abundant in the Chalk of Norfolk, and also in the Firestone of some localities: it is figured _Ly._ p. 212. Another small species, having the margin plicated (_O. plicata_), is also frequent in the Chalk. A large shell, with the margins deeply indented by angular folds, resembling the recent cockscomb oyster, is abundant in the Chalk Marl and Firestone; particularly near Dover, and around Selbourne in Hampshire, where it attracted the notice of White, by its resemblance to the living "Cockscomb Oyster" of the West Indies; it is named _Ostrea carinata_, and figured _Ly._ p. 212, _fig._ 204. One other species may be noticed, the Ostrea deltoidea, which has been found in every locality of the Kimmeridge Clay in England and France. It is a very flat species, and of a triangular form; the specific name is derived from a supposed resemblance to the Greek letter Δ, _delta_. I believe that in England no shells of this genus have been observed in strata older than the Lias. [Illustration: Lign. 127. Shells and Echinite from the Oolite and Lias. Fig. 1.--Trigonia clavellata. Oxford Cloy, _near Weymouth_. 2.--Trigonia gibbosa; a limestone cast. _Isle of Portland._ 3.--Cidaris Blumenbachii. _Oolite._ _Calne, Wilts._ 4.--Trigonia costata. _Oolite._ _Highworth, Wilts._ 5.--Spine of the Cidaris Blumenbachii. 6.--Gryphya incurva. Lias. _Cheltenham_. 7.--Ammonites Walcotii. Lias, _near Bath_. ] [Sidenote: GRYPHYÆA.]