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As to the Mollusca, which afford the most unbroken series of geological medals, the highest of that class, the Cephalopoda, abounded in older Silurian times, comprising several hundred species of chambered univalves.
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.
In fact, M. Defrance, in a work* full of new and ingenious ideas, not only recognizes this preponderance of the univalves in the number of the species, but also observes that out of 5500 fossil univalve, bivalve and multivalve shells, contained in his rich collections, there are 3066 univalve, 2108 bivalve, and 326 multivalve; the univalve fossils are therefore to the bivalve as three to two.
And it is noteworthy, first, that the defensive thorns which are permanent on the two thinner species, aculeatum and echinatum, disappear altogether on the thicker one, tuberculatum, as old age gives him a solid and heavy globose shell; and next, that he too, while young and tender, and liable therefore to be bored through by whelks and such murderous univalves, does actually possess the same briar- prickles, which his thinner cousins keep throughout life.
Several of the univalves had evidently long lain dead at the bottom of the sea, for their INSIDES were incrusted with Balani and Serpulae. All, according to Mr. G.B. Sowerby, are recent species: they consist of: Mytilus Magellanicus: same as that found at Valparaiso, and there stated to be probably distinct from the true M. Magellanicus of the east coast.
I soon became aware of the force of this rule in the class mollusca, when I first attempted to calculate the numerical proportion of Recent species in the Newer Pliocene formations as compared to the Older Pliocene, and of them again as contrasted with the Miocene; for it appeared invariably that a greater number of the lamellibranchs could be identified with living species than of the gasteropods, and of these last a greater number in the lower division, that of entire-mouthed univalves, than in that of the siphonated.
Most species of univalves are wanderers, many bivalves are free, and multivalves become fixed at an early stage of existence. The goose-necked barnacle, with its five valves, comes in its myriads attached to derelict coco-nuts, floating logs, and pumice-stone.
Paddling Crabs , with the hind pair of legs terminated by flattened plates to assist them in swimming, are brought up in the fishermen's nets. Hermit Crabs take possession of the deserted shells of the univalves, and crawl in pursuit of garbage along the moist beach.
It was also shown that the oyster, which supplied food to the primitive people, attained its full size in parts of the Baltic where it cannot now exist owing to a want of saltness in the water, and that certain marine univalves and bivalves, such as the common periwinkle, mussel, and cockle, of which the castaway shells are found in the mounds, attained in the olden time their full dimensions, like the oysters, whereas the same species, though they still live on the coast of the inland sea adjoining the mounds, are dwarfed and never half their natural size, the water being rendered too fresh for them by the influx of so many rivers.
The hard parts of the many animals which live upon the reef become imbedded in this coral limestone, so that a block may be full of shells of bivalves and univalves, or of sea-urchins; and even sometimes encloses the eggs of turtles in a state of petrifaction.