Gryphya. _Lign. 127, _fig._ 6._--The shells to which the term _Gryphæa_, or _Gryphites_, is applied, are related to the Oyster, but distinguished by the deep concave under valve, and its curved summit, or beak, and the almost flat, or opercular upper shell. The Gryphites are of a finer laminated structure than the oysters, and the ligament of the hinge is inserted in an elongated curved groove. There are about thirty British fossil species, none of which have been noticed below the Lias, in which formation one very remarkable species is so abundant as to be considered characteristic of the Liassic deposits. It is so faithfully represented, _Lign. 127, _fig._ 6_, that description is unnecessary. In the upper argillaceous beds of the Oolite and Kimmeridge Clay, a very small gryphite, (_G. virgula_, _Ly._ p. 260_) is so abundant, that it constitutes entire layers. The low cliffs on the west of Boulogne harbour, like those near Weymouth, are composed of this clay, and myriads of the gryphites are scattered on the shore, with other shells of the same deposits; these shelly beds are called _marnes à gryphées_, by the French geologists. A very large gryphite, _Gryphæa sinuata_, (_Min. Conch._ tab. 336,) is found in the Shanklin sand of the Isle of Wight, and of Kent and Sussex. At low water, in the sand along the shore under Dunnose Cliff, near Shanklin Chine, numerous specimens are always obtainable.[358] [358] The name _Exogyra_ was applied to the Chama-shaped species of Gryphæa by the late Mr. Sowerby, and other writers; but subsequent authors have included these shells in the present genus. [Sidenote: SPONDYLUS. PLAGIOSTOMA.] Spondylus. _Lign. 128._--A species of this genus is so frequent in the Chalk, that it ranks with certain Terebratulæ, as characteristic of that formation. One valve is covered with long slender spines, which, in the usual examples, are destroyed by the mode of extracting them. The specimen figured shows the appearance of a shell partly cleared; the remainder of the chalk might be removed by a penknife (taking care to leave the longest spines supported by brackets of chalk), and it would then resemble the beautiful fossils figured _Min. Conch._ tab. 78, and in _Geol. S. E._ p. 125. Between the beaks there is a triangular aperture in the spinous valve, which some naturalists, with much probability, suppose was once filled up with shell, as in the recent species. [Illustration: Lign. 128. Spondylus spinosus. _In Chalk-flint. Lewes._]