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In the remaining Molluscs the higher or more effective types are displacing the older. It is interesting to note that the oyster is fully developed, and has a very large kindred, in the Mesozoic seas. Among the Brachiopods the higher sloping-shoulder type displaces the square-shoulder shells.
A central tube, the SIPHUNCLE, passed through from the body chamber to the closed tip of the cone. The seashells, both brachiopods and mollusks, are in some respects the most important to the geologist of all fossils.
But the Arthrodirans were unwieldy and sluggish, and had to give way before more progressive types. The toothed shark gradually became the lord of the waters. The early shark ate, amongst other things, quantities of Molluscs and Brachiopods.
In others defensive armour is chiefly developed, and we get the lines of the heavy sluggish shell-fish, the Molluscs and Brachiopods, and, by a later compromise between speed and armour, the more active tough-coated Arthropods.
Between the Great and Inferior Oolite near Bath, an argillaceous deposit, called "the fuller's earth," occurs; but it is wanting in the north of England. The number of mollusca known in this deposit is about seventy; namely, fifty Lamellibranchiate Bivalves, ten Brachiopods, three Gasteropods, and seven or eight Cephalopods.
The Brachiopods and Molluscs still abound, but the Molluscs begin to outnumber the lower type of shell-fish. In the Cephalopods we find an increasing complication of the structure of the great spiral-shelled types. Such is the life of the Carboniferous period. The world rejoices in a tropical luxuriance.
Wood to the Palaeontographical Society, in which he has completed his figures and descriptions of the British crag shells of every age, list will be found of all the fossil shells, of which a summary is given in the table below. CHILLESFORD AND ALDEBY BEDS: Bivalves: 61 : 4. Univalves: 33 : 5. Brachiopods: 0 : 0. NORWICH OR FLUVIO-MARINE CRAG: Bivalves: 61 : 10. Univalves: 64 : 12.
"By the marked reduction in the Brachiopods compared with the now richly developed Gasteropods and and sinupalliate Lamellibranchs," it writhed and twisted before his dizzy eyes.
Among the Mollusks, Brachiopods are still prominent, one new genus among them, the Productus, being very remarkable on account of the manner in which one valve rises above the other.
Some of these animals have eye-specks on the edge of the mantle; but this is not a constant feature. This class of Acephala includes all the Oysters, Clams, Mussels, and the like. When named with reference to their double shells, they are called Bivalves; and with them are associated a host of less conspicuous animals, known as Ascidians, Brachiopods, and Bryozoa.
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