marked by lozenge-shaped cicatrices or scars, which are the points of attachment of the bases of the leaf-stalks. From the chalk-marl at Bonchurch, Isle of Wight; discovered by This fossil is the summit of a stem garnished with petioles or leaf-stalks on the upper part: from the lower portion the petioles have been removed. The internal axis is seen at a. The original is fifteen inches in length, and eleven and a half in the widest transverse diameter. On the other side of the specimen there appeared to me indications of the foliage, but M. Brongniart suggests it as more probable that they are only elongated and flattened petioles. The occurrence of this very characteristic plant of the Wealden high up in the chalk formation, is in accordance with the discovery of the bones of the iguanodon in the greensand of Kent.* In the strata of Tilgate Forest the remains of the clathrariæ are invariably associated with water-worn bones of reptiles, and pebbles, gravel, and other drift, and are often imbedded in the fluviatile conglomerate which occurs in some parts of the Wealden.† They appear to have floated down the river with the carcases and limbs of the reptiles, and at length to have sunk to the bottom, and become imbedded in the mud and sand of the delta. FOSSIL SHELLS.-The shells of the Wealden of the Isle of Wight belong to the same genera and species of fluviatile mollusca as those in the contemporaneous deposits of the south-east of England; and though spread over areas of considerable extent, and through deposits of great thickness, they comprise but a few species-a characteristic feature of the fauna of all rivers. The substance of the shells in the limestones is, for the most part, changed into carbonate of lime; but the shells in the argillaceous strata are often but little altered, and sometimes occur in as perfect a state as in very recent deposits; the epidermis and even the horny ligament being preserved. In the coarse limestones the shells have generally perished, and casts only remain. SUSSEX AND PETWORTH MARBLE.-The most abundant univalve shells are four or five kinds of Paludina; a common river snail, of which there are species in the tertiary freshwater strata of Headon Hill and Calbourne. (See Pl. I. fig. 6.) Numerous beds of marble, coarse limestone, clays, and shales, abound in or are wholly made up of paludinæ and minute crustaceans. The compact Sussex * "Wonders of Geology," vol. i. p. 394. + See remarks on the "Discovery of the Hylaosaurus," in "Wonders of Geology," vol. i. p. 401. limestone, which, from its taking a good polish, is called marble, principally consists of the shells of one large species, the Paludina fluviorum (Pl. VI. fig. 3), held together by an infiltration of crystallised carbonate of lime; the cavities of the shells, and the interstices between them, being often filled with white calc-spar. Upon examining thin slices of this marble under the microscope, the shells are found to contain myriads of the cases of Cyprides (lign. 25). The Purbeck marble differs from the above in being composed of a much smaller species-the Paludina elongata. (Pl. VI. fig. 2.) These marbles were in great repute with the architects of the middle ages, and there are but few of our cathedrals and ancient churches which do not still contain examples of both varieties, either in their columns, sepulchral monuments, or pavements. The polished cluster-columns of the Temple Church in London, and many of the monuments in Westminster Abbey, are of Purbeck marble; in other words, they are composed of the petrified shells of snails, that lived and died in the rivers which flowed through the country inhabited by the extinct colossal reptiles. The dark masses and veins so common in these marbles, are the remains of the bodies of the mollusca, changed into the carbonaceous substance termed molluskite. (See ante, p. 181.) The shells which were empty at the period of their becoming imbedded, had their cavities filled with the mud, silt, &c., which are now clay, marl, and limestone; but in the shells containing the animals, the gelatinous parts were converted into molluskite. In polished sections of the marbles this substance appears either within the shells, or in black or dark brown masses and veins. The most beautiful slabs of Sussex marble owe their variegated markings to the contrast produced by the black molluskite with the white calcareous spar. POTAMIDES.-Some species of the elongated, spiral, freshwater univalves, termed Potamides, of which several occur in profusion in the strata at Headon Hill (see Pl. II. figs. 2, 8, 9), are found in the Wealden. A very characteristic shell, the Potamides carbonarius, is figured in Pl. VI. fig. 5. This species was first discovered in the Weald clay at Pounceford in Sussex.* It also occurs in Hanover, in the clays intercalated with beds of coal, belonging to the Wealden formation. FOSSIL FRESHWATER BIVALVES.—Many of the bluish grey argillaceous limestones, as we have before remarked, are chiefly composed of five or six species of bivalves of the genera Cyclas and Cyrena; a small slab, with several shells in relief, is represented Pl. VI. fig. 4. Large blocks covered with shells of this kind are commonly to be found along the base of the cliffs, at Brook, Compton, and Sandown bays; and in the clays there are numerous seams of crushed shells of these genera. The hard grits often abound in casts of cyclades, associated with a small species of river-mussel (Unio); and some layers of Sussex marble contain an intermixture of these shells with the paludinæ. UNIO VALDENSIS. Pl. VI. fig. 1.-The only other bivalves which occur in any considerable number in the Wealden, are the mussels termed Unionido-a family of testaceous mollusca containing several genera, and comprising many species of considerable magnitude. These animals have a solid pearly shell, with two principal and two lateral teeth on the hinge. In number, variety, and internal lustre, the uniones of the large rivers of North America present a striking contrast with the few and homely British species. In a fossil state uniones are rare; a few occur in the carboniferous strata, and these, with eight or nine small species from the Wealden, figured by Dr. Fitton and myself, comprise all that are known in our secondary strata. * "Wonders of Geology," p. 350; it is there named Melanopsis. Many of the slabs of limestone procured from Sandown Bay, and used in the pavements at Brading, Ryde, &c., display sections of a species of unio, which is common in some of the Petworth and Purbeck beds. On a visit to Brook Point in 1843, I discovered a remarkably fine and large species of unio in the cliff above the petrified trees, and have since collected many specimens of this interesting fossil, which I have proposed to distinguish by the name of Unio Valdensis, or Wealden river-mussel. A small and young example is figured in Pl. VI. fig. 1; the hinge and the anterior muscular imprints are shown in the upper figure. The specimens obtained from the cliffs are in an excellent state of preservation; the surface of the shell, which is but little worn, is of a tawny-red colour, like some of the recent Ohio species. Even the horny ligament with its transverse rugæ remains; and in some instances the shell is occupied by the body of the animal in the state of molluskite. The constituent substance of the shell is occasionally changed into calcareous spar, and admits of a fine polish. Nodules of crystallised sulphate of barytes of a pink colour, are not uncommon within the shells. Some examples which I extracted from the bed containing the fossil trees, are partially invested with lignite. One pair of valves has been separated, and the hard grit with which they were filled cleared away, so as to develop the characters of the hinge, &c. The shell is of an ovate form, from four to six inches long, three inches in altitude, and two inches in thickness or diameter; the weight of a pair of valves cleared from extraneous matter is eleven ounces. The shell is equivalved, sub-equilateral, the posterior half one-fifth longer than the anterior, and compressed along the margin of the anterior slope. It is very thick and strong, and marked externally with longitudinal striæ. The summit is rounded, antero-dorsal, and slightly inclined forward; the umbones are decorticated, as in most of the shells of the Unionidæ. The ligament is external, post-apicial, and marked with transverse rugæ. |