3 5 6 LIGN 91. SHELLS, AND ECHINITE FROM THE COLITE AND LIAS. (Drawn by Mr. Joseph Dinkel.) Fig. 1.-TRIGONIA CLAVELLATA. Oxford Clay: near Weymouth. 2. TRIGONIA COSTATA; a limestone cast. Isle of Portland. 3.-CIDARIS BLUMENBACHII. Oolite. Calne, Wilts. 4. TRIGONIA COSTATA. Oolite. Highworth, Wills. 6.-GRYPHEA INCURVA. Lias. Cheltenham. Lias, near Bath. PLAGIOSTOMA. VIBRACY 389 Clay, a very small gryphite, (G. virgula, Ly. II. p. 48,) 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, Gryphaa 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 ShanklinChine, numerous specimens are always obtainable. - PLAGIOSTOMA. Lign. 92. 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 pen-knife (taking care to leave the longest spines supported by brackets of chalk), and it would then resemble the beautiful fossils figured Min. Conch. * The name Exogyra was applied to several species of Gryphæa by the late Mr. Sowerby, and other writers; but subsequent authors have included these shells in the present genus. 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 filled up in a recent state with cartilage, and that the original was a true Spondylus. In the present state of fossil conchology, and in an elementary work like the present, it appears to me desirable to retain the well-known name, originally imposed by the celebrated Lhywd. In the cretaceous strata of North America, Dr. Morton has discovered a Plagiostoma (P. dumosum) very nearly related to P. spinosum; but it differs in its general form, and has both valves beset with strong spines. There are several species of the genus without spines in the Chalk,* Oolite, and Lias. A very large species (P. giganteum), sometimes ten inches in diameter, abounds in the Lias (Ly. II. p. 60.). It is somewhat depressed in form, with the surface slightly striated; each valve has a pointed beak, with two lateral expansions, or ears, as they are termed by conchologists. Of the SPONDYLUS (Clam-shell), so well known in a recent state, but one species has been recognised in the British strata; but it is probable that some of the Plagiostoma, Ostreæ, &c. may belong to the Spondylidæ. I have the fragment of a large bivalve from the Kentish Rag (Mr. Bensted's quarry), which has the peculiar structure of the Water-clam (Spondylus varius of Mr. Broderip); namely, hollow interspaces formed by shelly layers or partitions, which were secreted by the posterior part of the mantle, or investing integument of the animal, as it gradually receded from that part of the shell. In the recent Water-clam the cells are full of fluid.† PLICATULA, is another genus of this family, of which there are three British fossil species. A delicate shell, with slender depressed spines (P. inflata. Foss. South D. Tab. XXVI.), occurs in the Chalk * See Foss. South Downs, Plate XXVI. Marl. The recent species are natives of the seas of warm climates. PECTEN. The common scallop-shell will serve as a type of this genus. The animals of these shells, unlike the oysters, have the power of locomotion, and when in the water, may be seen moving with rapidity, and flapping their shells to and fro with great activity. Numerous species are found fossil. In the Pliocene, and other marine tertiary deposits, Pectens abound; in the White Chalk there are several elegant forms (see Foss. South D. Plate XXV.); many kinds in the Oolite and Lias; and several in the Devonian strata. A large Mediterranean species (Pecten Jacobæus, Ly. I. p. 296.) occurs in the Pliocene strata of Palermo, in every stage of growth, and as perfect as if recent. The Chalk and Shanklin Sand contain a small inequivalved Pecten, the lower valve of which is convex, and pentangular, the upper flat, and both strongly ribbed, or pectinated; it is named, Pecten quinquecostatus (Foss. South D. Pl. XXVI. Ly. I. p. 403.); and in the cretaceous strata of North America a variety of this species is found. In the Chalk Marl a large and beautiful Pecten (P.Beaveri. Min. Conch. tab. 158.) is very common, and I have obtained from Hamsey and Southerham, examples in the most perfect state of preservation; it is a characteristic shell of the Chalk Marl of England (Foss. South D. Plate XXV. fig. 11.). |